Power generation unit, electronic apparatus, transportation device, and method of controlling power generation unit

ABSTRACT

A power generation unit includes a deforming member (a beam) adapted to deform while switching a deformation direction, a first piezoelectric device provided to the deforming member (the beam), a second piezoelectric device provided to the deforming member (the beam), an inductor electrically connected to the first piezoelectric device, a switch disposed between the first piezoelectric device and the inductor, and a control section adapted to detect a voltage generated in the second piezoelectric device, and if the voltage detected has a level one of equal to and higher than a predetermined level, electrically connect the first piezoelectric device and the inductor to each other using the switch.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/633,190, filed Oct. 2, 2012 which claims priority to Japanese PatentApplication No. 2011-218989 filed on Oct. 3, 2011 and Japanese PatentApplication No. 2011-219333 filed on Oct. 3, 2011, both of which areexpressly incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a power generation unit, an electronicapparatus, a transportation device, and a method of controlling thepower generation unit.

2. Related Art

When a piezoelectric material such as lead zirconium titanate (PZT),quartz crystal (SiO₂), or zinc oxide (ZnO) is deformed in response to anexternal force, electrical polarization is induced inside the material,and positive and negative charges appear on the surfaces. Such aphenomenon is called a so-called piezoelectric effect. There has beenproposed an electrical power generation method of vibrating a cantileverto thereby make a weight repeatedly act on the piezoelectric material,and thus taking out the charge generated on the surface of thepiezoelectric material as electricity using such a characteristicprovided to the piezoelectric material.

For example, by vibrating a metal cantilever having a mass disposed atthe tip and a thin plate made of the piezoelectric material bondedthereto, and taking out the positive and negative charges alternatelygenerated on the piezoelectric material due to the vibration, analternating current is generated. The alternating current is rectifiedusing diodes, and then stored in a capacitor, and then taken out aselectricity. Such a technology has been proposed in JP-A-7-107752. Therehas been also proposed a technology of arranging that a junction isclosed only during the period in which the positive charges aregenerated in a piezoelectric device to thereby make it possible toobtain a direct current without causing a voltage loss in the diodes(JP-A-2005-312269). Since it is possible to miniaturize the powergeneration unit by using these technologies, there is expected anapplication of, for example, incorporating the power generation unit in,for example, a small-sized electronic component instead of a battery.

However, in the proposed technology according to the related art, therearises a problem that the obtainable voltage is limited up to thevoltage generated by the electrical polarization of the piezoelectricmaterial. Therefore, in most cases, an additional step-up circuit isrequired, and there arises a problem that it is difficult tosufficiently miniaturize the power generation unit.

SUMMARY

An advantage of some aspects of the invention is to provide a technologycapable of generating a high voltage without growing the powergeneration unit using the piezoelectric effect of the piezoelectricmaterial in size.

The can be implemented as the following forms or application examples.

APPLICATION EXAMPLE 1

A power generation unit according to this application example of theinvention includes a deforming member adapted to deform while switchinga deformation direction, a first piezoelectric device provided to thedeforming member, a second piezoelectric device provided to thedeforming member, an inductor electrically connected to the firstpiezoelectric device, a switch disposed between the first piezoelectricdevice and the inductor, and a control section adapted to detect acurrent generated in the second piezoelectric device, and if the currentdetected has a level one of equal to and higher than a predeterminedlevel, electrically connect the first piezoelectric device and theinductor to each other via the switch.

According to this application example, since the first piezoelectricdevice and the second piezoelectric device are provided to the deformingmember, if the deforming member deforms, the first piezoelectric deviceand the second piezoelectric device are also deformed. As a result,positive and negative charges are generated in the piezoelectric devicesdue to the piezoelectric effect. The generation amount of the chargeincreases as the deformation amount of the piezoelectric deviceincreases. The first piezoelectric device constitutes the resonantcircuit together with the inductor, and the resonant circuit is providedwith the switch. The deformation of the deforming member is started inthe state in which the conduction in the switch is cut, and the switchis set to the conductive state when the deformation amount reaches anextreme value (i.e., the deformation direction is switched). Since thefirst piezoelectric device is deformed together with the deformingmember, and generates more charge as the deformation amount increases,when the charge generated in the first piezoelectric device reaches themaximum value, the first piezoelectric device is connected to theinductor to thereby form the resonant circuit. Then, the chargesgenerated in the first piezoelectric device flows into the inductor.Since the first piezoelectric device and the inductor constitute theresonant circuit, the current having flown into the inductor overshoots,and then flows into the terminal on the opposite side of the firstpiezoelectric device. This period (i.e., the period until the chargeflown out from one terminal of the first piezoelectric device flowsagain into the first piezoelectric device through the opposite sideterminal via the inductor) is a half of the resonance period of theresonant circuit composed of the first piezoelectric device and theinductor. Therefore, by forming the resonant circuit by setting theswitch to the connected state when the deformation direction of thefirst piezoelectric device is switched, and then setting the switch tothe disconnected state when the time period half as long as theresonance period has elapsed, it is possible to reverse the locations ofthe positive and negative charges having been generated in the firstpiezoelectric device before connecting the inductor. Then, whendeforming the deforming member in turn in the opposite direction fromthat state, since the first piezoelectric device is deformed in theopposite direction, the charges are accumulated in the firstpiezoelectric device in such a manner that the new charges furthergenerated by the piezoelectric effect in the state in which thelocations of the positive and negative charges are reversed are stackedincrementally thereon. Since the voltage generated also increases as thecharges are stored in the first piezoelectric device, it is possible togenerate a voltage higher than the voltage generated by the electricalpolarization of the piezoelectric material forming the firstpiezoelectric device without additionally preparing a step-up circuit.Further, in order to efficiently accumulate the charges in the firstpiezoelectric device in such a manner, it is important to connect theswitch when the deformation direction of the first piezoelectric deviceis switched to thereby form the resonant circuit. Here, since the firstpiezoelectric device and the second piezoelectric device are provided tothe deforming member, if the deformation direction of the firstpiezoelectric device is switched, the deformation direction of thesecond piezoelectric device is also switched. The timing at which thedeformation direction of the second piezoelectric device is switchedcoincides with the timing (the timing at which the current vanishes) atwhich the direction of the current due to the charge generated by thesecond piezoelectric device is switched. Therefore, by detecting thecurrent generated in the second piezoelectric device, the switch caneasily be set to the conductive state at the timing at which thedeformation direction of the deforming member is switched. The controlsection sets the switch to the conductive state for a predeterminedperiod of time started from the switching of the deformation directionof the first piezoelectric device to thereby make it possible toefficiently store the charges in the first piezoelectric device.Therefore, it is possible to realize the small-sized power generationunit capable of efficiently generating a high voltage using thepiezoelectric effect.

APPLICATION EXAMPLE 2

In the power generation unit according to the above application example,it is preferable that the control section electrically connects theswitch at a timing at which a deformation direction of the deformingmember is switched, and then electrically disconnects the switch at atiming at which a predetermined period has elapsed.

According to this application example, since theconnection/disconnection between the piezoelectric device and theinductor can periodically be repeated at appropriate timings in syncwith the deformation state (vibration state) of the deforming member byelectrically connecting (setting the switch to the conductive state) theswitch at the timing at which the deformation direction of the deformingmember is switched, it becomes possible to efficiently store the chargesin the piezoelectric device.

APPLICATION EXAMPLE 3

In the power generation unit according to the above application example,it is preferable that the control section includes a capacitor connectedin parallel to the second piezoelectric device, and a current detectcircuit adapted to detect a current flowing in the capacitor.

Since the capacitor is connected in parallel to the second piezoelectricdevice, the current having the same phase as that of the currentgenerated in the second piezoelectric device flows in the capacitor.Therefore, by detecting the current flowing in the capacitor, the timing(the timing at which the current vanishes) at which the direction of thecurrent due to the charge generated in the second piezoelectric deviceis switched can easily be detected.

APPLICATION EXAMPLE 4

In the power generation unit according to the above application example,it is preferable that the deforming member has a plurality of surfaces,the first piezoelectric device is provided to a first surface of thedeforming member, and the second piezoelectric device is provided to asecond surface of the deforming member different from the first surface.

Assuming that the first piezoelectric device and the secondpiezoelectric device are disposed on the same surface of the deformingmember, the installation area of the first piezoelectric device isreduced due to the presence of the second piezoelectric device. If thedeformation amount is the same, the larger the installation area of thepiezoelectric device is, the higher the power generation capacity is.Therefore, if it is arranged that the first piezoelectric device and thesecond piezoelectric device are disposed on the respective surfaces ofthe deforming member different from each other, the installation area ofthe first piezoelectric device can be increased, and therefore, thepower generation capacity of the power generation unit can be improved.

APPLICATION EXAMPLE 5

In the power generation unit according to the above application example,it is preferable that the deforming member has a plurality of surfaces,and the first piezoelectric device and the second piezoelectric deviceare provided to the same surface of the deforming member.

If the first piezoelectric device and the second piezoelectric deviceare disposed on the same surface of the deforming member, the firstpiezoelectric device and the second piezoelectric device can be providedto the deforming member at a time (in the same process). Therefore, itbecomes possible to manufacture the power generation unit with highproductivity.

APPLICATION EXAMPLE 6

In the power generation unit according to the above application example,it is preferable that the deforming member has an undeformablestationary end, and the second piezoelectric device is disposed at aplace closer to the stationary end of the deforming member than a placein the deforming member at which the first piezoelectric device isdisposed.

The bending moment of a part of the deforming member increases as thepart comes closer to the stationary end, and the deformation amount perunit length of the deforming member also increases in conjunctiontherewith. Therefore, by disposing the second piezoelectric device inthe vicinity of the stationary end, the sensitivity as the sensor isimproved, and the area of the second piezoelectric device can be reducedaccordingly. In addition, in the case of disposing the firstpiezoelectric device and the second piezoelectric device on the samesurface, the area of the first piezoelectric device for power generationcan be increased as much as the area which can be reduced from the areaof the second piezoelectric device, it becomes possible to prevent thedegradation of the power generation capacity caused by disposing thefirst piezoelectric device and the second piezoelectric device on thesame surface.

APPLICATION EXAMPLE 7

An electronic apparatus according to this application example is anelectronic apparatus using the power generation unit according to anyone of the application examples described above.

APPLICATION EXAMPLE 8

A transportation device according to this application example is atransportation device using the power generation unit according to anyone of the application examples described above.

According to these application examples of the invention, since it ispossible to incorporate the power generation unit in the electronicapparatus such as a remote controller instead of a battery, the powercan be generated due to the transportation of the electronic apparatus,and in addition, by using the power generation unit according to theinvention in a transportation device such as a vehicle or a electrictrain, it is also possible to generate electricity by the vibration dueto the transportation, and to efficiently supply the electricity to theequipment provided to the transportation device.

APPLICATION EXAMPLE 9

A method of controlling a power generation unit according to thisapplication example of the invention is directed to a method ofcontrolling a power generation unit including a deforming member adaptedto deform while switching a deformation direction, a first piezoelectricdevice provided to the deforming member, a second piezoelectric deviceprovided to the deforming member, an inductor electrically connected tothe first piezoelectric device, and a switch disposed between the firstpiezoelectric device and the inductor. The method includes detecting acurrent generated in the second piezoelectric device, and connecting thefirst piezoelectric device and the inductor electrically to each othervia the switch based on the detection result of the current.

According to this application example, since the first piezoelectricdevice and the second piezoelectric device are provided to the deformingmember, if the deformation direction of the first piezoelectric deviceis switched, the deformation direction of the second piezoelectricdevice is also switched. The timing at which the deformation directionof the second piezoelectric device is switched coincides with the timing(the timing at which the current vanishes) at which the direction of thecurrent due to the charge generated by the second piezoelectric deviceis switched. Therefore, by detecting the current generated in the secondpiezoelectric device, and then setting the switch to the conductivestate for a predetermined period of time based on the current detected,it becomes possible to efficiently accumulate the charges in the firstpiezoelectric device. Since the voltage generated by the firstpiezoelectric device also increases in accordance with the chargeaccumulated in the first piezoelectric device, it is possible togenerate a voltage higher than the voltage generated by the electricalpolarization of the piezoelectric material without additionallypreparing a step-up circuit.

APPLICATION EXAMPLE 10

A power generation unit according to this application example of theinvention includes a deforming member adapted to deform while switchinga deformation direction, a first piezoelectric device provided to thedeforming member, a second piezoelectric device provided to thedeforming member, and adapted to generate electrical power, an amount ofwhich is smaller than an amount of electrical power generated by thefirst piezoelectric device, an inductor electrically connected to thefirst piezoelectric device, a switch disposed between the firstpiezoelectric device and the inductor, and a control section adapted todetect a voltage generated in the second piezoelectric device, and ifthe voltage detected has a level one of equal to and higher than apredetermined level, electrically connect the first piezoelectric deviceand the inductor to each other using the switch.

According to this application example, since the first piezoelectricdevice and the second piezoelectric device are provided to the deformingmember in the power generation unit, if the deforming member deforms,the first piezoelectric device and the second piezoelectric device arealso deformed. As a result, positive and negative charges are generatedin the piezoelectric devices due to the piezoelectric effect. Thegeneration amount of the charge increases as the deformation amount ofthe piezoelectric device increases. The first piezoelectric deviceconstitutes the resonant circuit together with the inductor, and theresonant circuit is provided with the switch. The deformation of thedeforming member is started in the state in which the conduction in theswitch is cut, and the switch is set to the conductive state when thedeformation amount reaches an extreme value (i.e., the deformationdirection is switched). Since the first piezoelectric device (and thesecond piezoelectric device) is deformed together with the deformingmember, and generates more charge as the deformation amount increases,when the charge generated in the first piezoelectric device (and thesecond piezoelectric device) reaches the maximum value, the firstpiezoelectric device is connected to the inductor to thereby form theresonant circuit. Then, the charges generated in the first piezoelectricdevice flows into the inductor. Since the first piezoelectric device andthe inductor constitute the resonant circuit, the current having flowninto the inductor overshoots, and then flows into the terminal on theopposite side of the first piezoelectric device. This period (i.e., theperiod until the charge flown out from one terminal of the firstpiezoelectric device flows again into the first piezoelectric devicethrough the opposite side terminal via the inductor) is a half of theresonance period of the resonant circuit composed of the firstpiezoelectric device and the inductor. Therefore, by forming theresonant circuit by setting the switch to the connected state when thedeformation direction of the first piezoelectric device is switched, andthen setting the switch to the disconnected state when the time periodhalf as long as the resonance period has elapsed, it is possible toreverse the locations of the positive and negative charges having beengenerated in the first piezoelectric device before connecting theinductor. Then, when deforming the deforming member in turn in theopposite direction from that state, since the first piezoelectric deviceis deformed in the opposite direction, the charges are accumulated inthe first piezoelectric device in such a manner that the new chargesfurther generated by the piezoelectric effect in the state in which thelocations of the positive and negative charges are reversed are stackedincrementally thereon. Since the voltage generated also increases as thecharges are stored in the first piezoelectric device, it is possible togenerate a voltage higher than the voltage generated by the electricalpolarization of the piezoelectric material forming the firstpiezoelectric device without additionally preparing a step-up circuit.Further, in order to efficiently accumulate the charges in the firstpiezoelectric device in such a manner, it is important to connect theswitch when the deformation direction of the first piezoelectric deviceis switched to thereby form the resonant circuit. Here, since the firstpiezoelectric device and the second piezoelectric device are provided tothe deforming member, if the deformation direction of the firstpiezoelectric device is switched, the deformation direction of thesecond piezoelectric device is also switched. Since the larger thedeformation amount is, the higher voltage the second piezoelectricdevice generates, at the position where the deformation direction of thesecond piezoelectric device is switched, the voltage generated by thesecond piezoelectric device takes an extreme value. Therefore, bydetecting the voltage generated in the second piezoelectric device, andthen setting the switch to the conductive state for a predeterminedperiod of time from the time point when the voltage takes the extremevalue, it becomes possible to efficiently accumulate the charges in thefirst piezoelectric device. As a result, it becomes unnecessary toprovide an additional step-up circuit, and thus it becomes possible toobtain a small-sized and highly efficient power generation unit.

APPLICATION EXAMPLE 11

In the power generation unit according to the above application example,it is preferable that the first piezoelectric device is higher inpiezoelectric constant than the second piezoelectric device.

In general, the higher the piezoelectric constant of the piezoelectricdevice is, the higher the power generation capacity is. Therefore, bysetting the piezoelectric constant of the first piezoelectric device tobe higher than that of the second piezoelectric device, the voltagehigher than the voltage of the second piezoelectric device is generatedby the first piezoelectric device used for supplying the charge to theoutside, and the power generation capacity of the power generation unitcan be improved.

APPLICATION EXAMPLE 12

In the power generation unit according to the above application example,it is preferable that the first piezoelectric device is larger in areaof a part capable of generating electrical power than the secondpiezoelectric device.

In the piezoelectric devices having the equivalent power generationcapacity, the larger the area of the part capable of generating theelectrical power, the higher the power generation capacity is.Therefore, by setting the area of the part capable of generating theelectrical power of the first piezoelectric device to be larger thanthat of the second piezoelectric device, the voltage higher than thevoltage of the second piezoelectric device is generated by the firstpiezoelectric device used for supplying the charge to the outside, andthe power generation capacity of the power generation unit can beimproved.

APPLICATION EXAMPLE 13

In the power generation unit according to the above application example,it is preferable that the number of the first piezoelectric devices isplural.

The larger the plural number of the first piezoelectric devices is, themore the power generation amount becomes. Therefore, by providing aplurality of first piezoelectric devices, a higher voltage than thevoltage of the second piezoelectric device is generated from the firstpiezoelectric device used for supplying the charge to the outside, andthe power generation capacity of the power generation unit can beimproved.

APPLICATION EXAMPLE 14

In the power generation unit according to the above application example,it is preferable that the deforming member has a plurality of surfaces,the first piezoelectric device is provided to a first surface of thedeforming member, and the second piezoelectric device is provided to asecond surface of the deforming member different from the first surface.

Assuming that the first piezoelectric device and the secondpiezoelectric device are disposed on the same surface of the deformingmember, the installation area of the first piezoelectric device isreduced due to the presence of the second piezoelectric device. If thedeformation amount is the same, the larger the installation area of thepiezoelectric device is, the higher the power generation capacity is.Therefore, if it is arranged that the first piezoelectric device and thesecond piezoelectric device are disposed on the respective surfaces ofthe deforming member different from each other, the installation area ofthe first piezoelectric device can be increased, and therefore, thepower generation capacity of the power generation unit can be improved.

APPLICATION EXAMPLE 15

In the power generation unit according to the above application example,it is preferable that the first piezoelectric device and the secondpiezoelectric device are provided to the same surface of the deformingmember.

If the first piezoelectric device and the second piezoelectric deviceare disposed on the same surface of the deforming member, the firstpiezoelectric device and the second piezoelectric device can be providedto the deforming member at a time (in the same process). Therefore, itbecomes possible to manufacture the power generation unit with highproductivity.

APPLICATION EXAMPLE 16

In the power generation unit according to the above application example,it is preferable that the first piezoelectric device and the secondpiezoelectric device are equal in length in a longitudinal direction toeach other.

If the first piezoelectric device and the second piezoelectric deviceare equal in length in the longitudinal direction to each other, thefirst piezoelectric device and the second piezoelectric device aredeformed in sync with the deformation of the deforming member in thelongitudinal direction. Therefore, when the deformation direction of thefirst piezoelectric device is switched, the deformation direction of thesecond piezoelectric device is also switched in sync therewith, andtherefore, the timing at which the deformation direction of the firstpiezoelectric device is switched and the timing at which the deformationdirection of the second piezoelectric device is switched roughlycoincide with each other. As a result, accurate switching of the switchbecomes possible, and the power generation unit with high powergeneration efficiency can be provided as described above.

APPLICATION EXAMPLE 17

In the power generation unit according to the above application example,it is preferable that the deforming member has an undeformablestationary end, and the second piezoelectric device is disposed at aplace closer to the stationary end of the deforming member than a placein the deforming member at which the first piezoelectric device isdisposed.

In the beam configured including the undeformable stationary end, thebending moment of a part increases as the part moves from the tip (thefree end) and comes closer to the stationary end (the base), and thedeformation amount of the beam per unit length also increases inconjunction therewith. Therefore, by disposing the second piezoelectricdevice used for the control in the vicinity of the stationary end, thesensitivity as the sensor is improved, and the width of the secondpiezoelectric device used for the control can be reduced accordingly. Asa result, since the area of the first piezoelectric device used for thepower generation can be increased, it becomes possible to suppress thedegradation of the power generation capacity caused by disposing thefirst piezoelectric device used for the power generation and the secondpiezoelectric device used for the control on the same surface.

APPLICATION EXAMPLE 18

An electronic apparatus according to this application example is anelectronic apparatus using the power generation unit according to anyone of the application examples described above.

APPLICATION EXAMPLE 19

A transportation device according to this application example is atransportation device using the power generation unit according to anyone of the application examples described above.

According to these aspects of the invention, since it is possible toincorporate the power generation unit in a remote controller or aportable electronic apparatus instead of a battery, the power can begenerated due to the transportation of the electronic apparatus, and inaddition, by using the power generation unit according to theapplication example of the invention in a transportation device such asa vehicle or a electric train, it is also possible to generateelectricity by the vibration due to the transportation, and toefficiently supply the electricity to the equipment provided to thetransportation device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1A is an explanatory diagram showing a structure of a powergeneration unit according to a first embodiment of the invention.

FIG. 1B is an explanatory diagram showing the structure of the powergeneration unit according to the first embodiment of the invention.

FIGS. 2A through 2D are explanatory diagrams showing an operation of thepower generation unit according to the first embodiment.

FIGS. 3A through 3F are explanatory diagrams conceptually showing ananterior half of the operation principle of the power generation unitaccording to the first embodiment.

FIGS. 4A through 4F are explanatory diagrams conceptually showing aposterior half of the operation principle of the power generation unitaccording to the first embodiment.

FIG. 5A is an explanatory diagram showing the reason that it is possibleto raise the voltage between terminals of a piezoelectric element evenif a switch is set to the ON state at an arbitrary timing.

FIG. 5B is an explanatory diagram showing the reason that it is possibleto raise the voltage between the terminals of the piezoelectric elementeven if the switch is set to the ON state at an arbitrary timing.

FIG. 6 is an explanatory diagram showing the reason that it is possibleto raise the voltage between the terminals of the piezoelectric elementeven if the switch is set to the ON state at an arbitrary timing.

FIG. 7A is an explanatory diagram showing the reason that it is possibleto raise the voltage between the terminals of the piezoelectric elementeven if the switch is set to the ON state at an arbitrary timing.

FIG. 7B is an explanatory diagram showing the reason that it is possibleto raise the voltage between the terminals of the piezoelectric elementeven if the switch is set to the ON state at an arbitrary timing.

FIG. 8 is a diagram showing a voltage waveform between the terminals ofthe piezoelectric element in the case of setting the switch to the ONstate for a period of time three halves as long as the resonance periodof an LC resonant circuit.

FIG. 9 is a diagram showing a voltage waveform between the terminals ofthe piezoelectric element in the case of setting the switch to the ONstate for a period of time a quarter as long as the resonance period ofthe LC resonant circuit.

FIGS. 10A through 10C are explanatory diagrams showing the reason thatthe switch SW can be controlled at an appropriate timing by detecting acurrent generated in a controlling piezoelectric device.

FIG. 11 is a flowchart for explaining a switch control process as anexample of a method of controlling the power generation unit accordingto the embodiment.

FIG. 12 is a block diagram showing an example of a configuration of acurrent detect circuit.

FIG. 13A is an explanatory diagram showing a power generation unitaccording to a second embodiment provided with a plurality ofcontrolling piezoelectric devices.

FIG. 13B is an explanatory diagram showing the power generation unitaccording to the second embodiment provided with the plurality ofcontrolling piezoelectric devices.

FIG. 13C is an explanatory diagram showing the power generation unitaccording to the second embodiment provided with the plurality ofcontrolling piezoelectric devices.

FIG. 14 is a circuit diagram showing an electrical structure of thepower generation unit according to a third embodiment.

FIGS. 15A through 15C are explanatory diagrams showing the reason thatthe switch SW can be controlled at an appropriate timing by detecting avoltage generated in a controlling piezoelectric device.

FIG. 16 is a flowchart for explaining a switch control process as anexample of a method of controlling the power generation unit accordingto the embodiment.

FIG. 17 is a circuit diagram showing an electrical structure of a powergeneration unit according to a fourth embodiment provided with aplurality of controlling piezoelectric devices.

FIG. 18A is an explanatory diagram showing a first modified exampleprovided with a power-generating piezoelectric device and twocontrolling piezoelectric devices.

FIG. 18B is an explanatory diagram showing the first modified exampleprovided with the power-generating piezoelectric device and the twocontrolling piezoelectric devices.

FIG. 19 is an explanatory diagram showing a second modified examplehaving a power-generating piezoelectric device and a controllingpiezoelectric device disposed on the same surface of a beam.

FIG. 20 is an explanatory diagram showing another configuration of thesecond modified example having the power-generating piezoelectric deviceand the controlling piezoelectric device disposed on the same surface ofthe beam.

FIG. 21 is an explanatory diagram showing a third modified examplehaving a power-generating piezoelectric device and a plurality ofcontrolling piezoelectric devices disposed on the same surface.

FIG. 22 is an explanatory diagram showing another configuration of thethird modified example having the power-generating piezoelectric deviceand the plurality of controlling piezoelectric devices disposed on thesame surface.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, preferred embodiments of the invention will be described indetail with reference to the accompanying drawings. The drawings usedtherein are for the sake of convenience of explanation. The embodimentsdescribed below do not unreasonably limit the content of the inventionas set forth in the appended claims. Further, all of the constituentsdescribed below are not necessarily essential elements of the invention.

Hereinafter, embodiments of the invention will be explained along thefollowing procedure to thereby clarify the content of the inventiondescribed above.

A. First Embodiment

-   -   A-1. Structure of Power Generation Unit    -   A-2. Operation of Power Generation Unit    -   A-3. Operation Principle of Power Generation Unit    -   A-4. Switching Timing of Switch

B. Second Embodiment

C. Third Embodiment

D. Fourth Embodiment

E. Modified Examples

-   -   E-1. First Modified Example    -   E-2. Second Modified Example    -   E-3. Third Modified Example

A. FIRST EMBODIMENT A-1. Structure of Power Generation Unit

FIGS. 1A and 1B are explanatory diagrams showing a structure of a powergeneration unit 100 according to the present embodiment. FIG. 1A shows amechanical structure of the power generation unit 100, and FIG. 1B showsan electrical structure thereof. The mechanical structure of the powergeneration unit 100 according to the present example is formed as acantilever structure in which the beam 104 having a mass 106 disposed atthe tip thereof is fixed to a base 102 on the base end side thereof, andthe beam 104 can deform while switching the deformation direction. Thebase 102 is preferably fixed inside the power generation unit 100. Apiezoelectric device 108 and a piezoelectric device 110 are disposed onthe surface of the beam 104. The piezoelectric device 108 is configuredincluding a piezoelectric element 108 c formed of a piezoelectricmaterial such as lead zirconium titanate (PZT), and a first electrode(an upper electrode) 108 a and a second electrode (a lower electrode)108 b each formed of a metal thin film on the surface of thepiezoelectric element 108 c. The first electrode (the upper electrode)108 a and the second electrode (the lower electrode) 108 b are disposedso as to be opposed to each other across the piezoelectric element 108c. The piezoelectric device 110 is configured including a piezoelectricelement 110 c formed of a piezoelectric material such as lead zirconiumtitanate (PZT), and a first electrode (an upper electrode) 110 a and asecond electrode (a lower electrode) 110 b each formed of a metal thinfilm on the surface of the piezoelectric element 110 c. The firstelectrode (the upper electrode) 110 a and the second electrode (thelower electrode) 110 b are disposed so as to be opposed to each otheracross the piezoelectric element 110 c. Although the piezoelectricdevice 108 and the piezoelectric device 110 have the same shape in theexample shown in FIG. 1A, it is not necessary required for them to havethe same shape. For example, if the piezoelectric device 108 has themaximum installable length and the maximum installable width withrespect to the beam 104, a large power generation amount of thepiezoelectric device 108 is obtained. On the other hand, if thepiezoelectric device 110 has the minimum installable width (the lengththereof in the direction along the shorter dimension of the beam 104),the displacement resistance of the beam 104 caused by the piezoelectricdevice 110 is reduced, and therefore, the power generation efficiency isimproved. Further, since the piezoelectric device 108 and thepiezoelectric device 110 deform due to the deformation of the beam 104,the beam 104 corresponds to the “deforming member” according to theinvention.

Since the beam 104 is fixed to the base 102 at the base end sidethereof, and has the mass 106 disposed on the tip side thereof, when avibration or the like is applied to the beam 104, the tip of the beam104 vibrates with a large amplitude as indicated by the outlined arrowin the drawing. As a result, a compression force and a tensile forcealternately act on the piezoelectric device 108 and the piezoelectricdevice 110 disposed on the respective surfaces of the beam 104. Then,the piezoelectric element 108 c of the piezoelectric device 108generates positive and negative charges due to the piezoelectric effect,and the charges appear in the first electrode 108 a and the secondelectrode 108 b. Similarly, the piezoelectric element 110 c of thepiezoelectric device 110 generates positive and negative charges due tothe piezoelectric effect, and the charges appear in the first electrode110 a and the second electrode 110 b. Although the mass 106 is notessential, it is desirable to create imbalance in mass between the tipside and the base end side of the beam 104. This is because thedisplacement of the beam 104 becomes easy to repeat in response to onevibration, for example, due to the imbalance in mass.

FIG. 1B shows an example of a circuit diagram of the power generationunit 100 according to the present embodiment. The piezoelectric element108 c of the piezoelectric device 108 can electrically be expressed as acurrent source and a capacitor (a capacitive component) Cg for storingcharges. The piezoelectric element 110 c of the piezoelectric device 110can electrically be expressed as a current source and a capacitor (acapacitive component) Cs for storing charges. The inductor L isconnected in parallel to the piezoelectric element 108 c, andconstitutes a resonant circuit including the piezoelectric device 108.In other words, the inductor L is electrically connected to thepiezoelectric device 108, and constitutes an electrical resonant circuittogether with the capacitive component Cg of the piezoelectric element108 c. Further, the resonant circuit is provided with a switch SW forswitching ON/OFF the resonant circuit connected in series to theinductor L.

The control section 130 controls to set the switch SW to the ON/OFFstates. Specifically, the control section 130 detects the currentgenerated in the piezoelectric device 110, and if the current detectedhas a value equal to or higher than a predetermined value, the controlsection 130 set the switch SW to the conductive state to therebyelectrically connect the piezoelectric device 108 and the inductor L toeach other via the switch SW. In the present embodiment, the controlsection 130 is configured including a capacitor 132 connected inparallel to the piezoelectric device 110, a current detect circuit 134for detecting the current flowing through the capacitor 132, and acontrol circuit 136 for controlling the switch SW based on the currentdetected by the current detect circuit 134. The control circuit 136 canalso be formed of a central processing unit (CPU). Details of theoperation of the control section 130 will be described later.

The first electrode 108 a and the second electrode 108 b provided to thepiezoelectric element 108 c of the piezoelectric device 108 areconnected to a rectifier 120 for rectifying the current generated by thepiezoelectric device 108. In the present embodiment, the rectifier 120is a full bridge rectifier composed of four diodes D1 through D4. Byforming the rectifier 120 with the full bridge rectifier, it is possibleto efficiently extract the charge generated by the piezoelectric device108 to thereby efficiently generate the electrical power. Further, acapacitor (an output capacitor) C1 for storing the current after therectification for driving an electrical load is connected to therectifier 120. In other words, the capacitor C1 is connected in parallelto the piezoelectric device 108 via the rectifier 120. The capacitor C1is not an essential constituent, and can be provided if need arises.

On the other hand, the piezoelectric device 110 is provided forcontrolling the switch SW, and the first electrode 110 a and the secondelectrode 110 b provided to the piezoelectric device 110 are connectedto the control section 130. Therefore, it is assumed hereinafter thatthe piezoelectric device 108 may be referred to as a “power-generatingpiezoelectric device,” and the piezoelectric device 110 may be referredto as a “controlling piezoelectric device.” The piezoelectric device 108corresponds to a “first piezoelectric device” according to theinvention, and the piezoelectric device 110 corresponds to a “secondpiezoelectric device” according to the invention.

A-2. Operation of Power Generation Unit

FIGS. 2A through 2D are explanatory diagrams showing the operation ofthe power generation unit 100 according to the present embodiment. FIG.2A shows how the displacement u of the tip of the beam 104 varies due tothe vibration of the beam 104. The positive displacement u representsthe state (the state in which the upper surface side of the beam 104 hasa concave shape) in which the beam 104 is warped upward, and thenegative displacement (−u) represents the state (the state in which thelower surface side of the beam 104 has a concave shape) in which thebeam 104 is warped downward. FIG. 2B shows the state of the currentgenerated by the piezoelectric element 108 c due to the deformation ofthe beam 104 and the electromotive force caused inside the piezoelectricelement 108 c as a result thereof. In FIG. 2B the state of the chargegenerated in the piezoelectric element 108 c is expressed as an amountof the charge (i.e., a current Ipzt) generated per unit time, and theelectromotive force generated in the piezoelectric element 108 c isexpressed as the voltage Vpzt generated between the first electrode 108a and the second electrode 108 b.

As described above with reference to FIG. 1A, the beam 104 is alsoprovided with the piezoelectric device 110, and when the beam 104deforms, the piezoelectric element 110 c also deforms similarly to thepiezoelectric element 108 c. Therefore, the current Ipzt2 similar to thecurrent Ipzt shown in FIG. 2B, and the voltage Vpzt2 similar to thevoltage Vpzt shown in FIG. 2B are also generated inside thepiezoelectric element 110 c in just the same manner as the piezoelectricelement 108 c.

As shown in FIGS. 2A and 2B, during the period in which the displacementof the beam 104 keeps increasing, the piezoelectric element 108 cgenerates a current in the positive direction (i.e., the current Ipzttakes a positive value), and the voltage Vpzt between the firstelectrode 108 a and the second electrode 108 b increases in the positivedirection in conjunction therewith. If the voltage Vpzt in the positivedirection exceeds the sum of the voltage VC1 of the capacitor C1 and atwofold of the forward voltage drop Vf of the diode constituting therectifier 120, namely VC1+2 Vf, the charge generated thereafter can betaken out as a direct current and stored in the capacitor C1. During theperiod in which the displacement of the beam 104 keeps decreasing, thepiezoelectric element 108 c generates a current in the negativedirection (i.e., the current Ipzt takes a negative value), and thevoltage Vpzt between the first electrode 108 a and the second electrode108 b increases in the negative direction in conjunction therewith. Ifthe voltage Vpzt in the negative direction exceeds the sum of VC1 and 2Vf of the rectifier 120, the charge generated can be taken out as adirect current and stored in the capacitor C1. In other words, even whenkeeping the switch SW shown in FIG. 1B in the OFF state, the charge canbe stored in the capacitor C1 regarding the part indicated by hatchingin FIG. 2B.

The amount of the charge (a power generation efficiency) which can betaken out from the piezoelectric element 108 c in a predetermined periodof time differs according to the timing at which the switch SW is set tothe ON state, and the power generation efficiency is maximized in thecase in which the switch SW is set to the ON state at the timing atwhich the deformation direction of the beam 104 is switched as shown inFIG. 2C. Hereinafter, the operation in the case in which the powergeneration efficiency is maximized will firstly be explained.

It is assumed that the control section 130 set the switch SW to the ONstate at the timing shown in FIG. 2C. Then, as shown in FIG. 2D, thereoccurs a phenomenon that the voltage waveform between the firstelectrode 108 a and the second electrode 108 b varies as if it isshifted at the moment that the switch SW is set to the ON state.Specifically, in the period B indicated as “B” in FIG. 2D, such avoltage waveform indicated by the thick dotted line as is obtained byshifting the voltage Vpzt indicated by the thin dotted linecorresponding to the electromotive force of the piezoelectric element108 c toward the negative side appears between the first electrode 108 aand the second electrode 108 b. The reason that such a phenomenon occurswill be described later. In the period C indicated as “C” in FIG. 2D,there appears such a voltage waveform indicated by the thick dotted lineas is obtained by shifting the waveform of the voltage Vpztcorresponding to the electromotive force of the piezoelectric element108 c toward the positive side. Similarly, thereafter, in each of theperiod D, the period E, the period F, and so on, there appears such avoltage waveform indicated by the thick dotted line as is obtained byshifting the waveform of the voltage Vpzt corresponding to theelectromotive force of the piezoelectric element 108 c toward thepositive side or the negative side. In the part (the part indicated byhatching in FIG. 2D) where the voltage waveform thus shifted exceeds thesum of VC1 and 2 Vf, the charge generated in the piezoelectric element108 c can be stored in the capacitor C1. As a result of the flow of thecharge from the piezoelectric element 108 c to the capacitor C1, thevoltage Vgen between the first electrode 108 a and the second electrode108 b is clipped at the voltage corresponding to the sum of VC1 and 2Vf. As a result, the waveform indicated by the thick solid line in FIG.2D is obtained as the voltage waveform of the voltage between the firstelectrode 108 a and the second electrode 108 b.

As is obvious from the comparison between the case of keeping the switchSW in the OFF state shown in FIG. 2B and the case of setting the switchSW to the ON state at the timing when the deformation direction of thebeam 104 is switched as shown in FIG. 2D, in the power generation unit100 according to the present embodiment, it becomes possible toefficiently store the charge in the capacitor C1 by setting the switchSW to the ON state at appropriate timings. Therefore, the powergeneration unit 100 according to the first embodiment is provided withthe controlling piezoelectric device 110 in order to set the switch SWto the ON state at appropriate timings, and detects the currentgenerated in the piezoelectric device 110 to control the switch SW. Thispoint will be explained later in detail.

If the charge is stored in the capacitor C1 and the inter-terminalvoltage of the capacitor C1 increases, the shift amount of the voltagewaveform also increases in accordance therewith. For example, incomparison between the period B (the state in which no charge is storedin the capacitor C1) in FIG. 2D and the period H (the state in which thecharge is stored a little bit in the capacitor C1) in FIG. 2D, the shiftamount of the voltage waveform is larger in the period H. Similarly, incomparison between the period C and the period I in FIG. 2D, the shiftamount of the voltage waveform is larger in the period I in which thecharge stored in the capacitor C1 is increased. Although the reason whysuch a phenomenon occurs will be described later, as a result, in thepower generation unit 100 according to the present embodiment, itbecomes also possible to store the voltage higher than the voltage Vpzt,which is generated between the first electrode 108 a and the secondelectrode 108 b by deforming the piezoelectric element 108 c, in thecapacitor C1. As a result, it becomes unnecessary to provide anadditional step-up circuit, and thus it becomes possible to obtain asmall-sized and highly efficient power generation unit.

A-3. Operation Principle of Power Generation Unit

FIGS. 3A through 3F are explanatory diagrams conceptually showing ananterior half of the operation principle of the power generation unit100 according to the present embodiment. FIGS. 4A through 4F areexplanatory diagrams conceptually showing a posterior half of theoperation principle of the power generation unit 100 according to thepresent embodiment. FIGS. 3A through 3F conceptually show the movementof the charge in the capacitor Cg when setting the switch SW1 to the ONstate in accordance with the deformation of the piezoelectric element108 c. FIG. 3A shows the state in which the piezoelectric element 108 c(the beam 104, to be precise) is deformed upward (so that the uppersurface side has a concave shape). If the piezoelectric element 108 c isdeformed upward, the current in the positive direction flows from thecurrent source, then the charge is stored in the capacitor Cg, and thevoltage in the positive direction is generated between the terminals ofthe piezoelectric element 108 c. The voltage value increases as thedeformation of the piezoelectric element 108 c increases. The switch SWis set to the ON state at the timing (the timing at which the amount ofthe charge reaches a peak; see FIG. 3B) at which the deformation of thepiezoelectric element 108 c reaches a peak.

FIG. 3C shows the state immediately after setting the switch SW to theON state. Since the charge is stored in the capacitor Cg, the charge isurged to flow into the inductor L. Although a magnetic flux is generated(the magnetic flux increases) when a current flows through the inductorL, the inductor L has a nature (a self-induction effect) of generating aback electromotive force in the direction of preventing the change inthe magnetic flux penetrating the inductor itself. Since the magneticflux is urged to increase due to the charge flowing therethrough whenthe switch SW is set to the ON state, the back electromotive forceoccurs in the direction (in other words, the direction of hindering theflow of the charge) of preventing the magnetic flux from increasing. Thelevel of the back electromotive force is proportional to the variationrate (the variation per unit time) of the magnetic flux. In FIG. 3C, theback electromotive force generated in the inductor L in such a manner asdescribed above is indicated by the arrow provided with hatching. Sincesuch a back electromotive force is generated, only a little amount ofthe charge in the piezoelectric element 108 c flows out when setting theswitch SW to the ON state. In other words, the current flowing throughthe inductor L increases only gradually.

Subsequently, when the current flowing through the inductor L reaches apeak value, the variation rate of the magnetic flux reaches “0,” andtherefore, the back electromotive force reaches “0” as shown in FIG. 3D.The current starts decreasing in turn. Then, since the magnetic fluxpenetrating the inductor L decreases, the electromotive force occurs inthe inductor L in the direction (the direction of urging the current toflow) of preventing the decrease in the magnetic flux (see FIG. 3E). Asa result, the current continues to flow through the inductor L whilepulling out the charge from the capacitor Cg due to the electromotiveforce. If no loss occurs during the migration of the charge, all thecharge generated by the deformation of the piezoelectric element 108 cis migrated, and there occurs the state (i.e., the state in which thepositive charge is distributed on the lower surface side of thepiezoelectric element 108 c, and the negative charge is distributed onthe upper surface side) looked as if the positive and negative chargesare replaced with each other. FIG. 3F shows the state in which all thecharge generated by the deformation of the piezoelectric element 108 chas been migrated.

If the switch SW is kept in the ON state without change, a conversephenomenon to the content described above occurs in turn. Specifically,the positive charge on the lower surface side of the piezoelectricelement 108 c is urged to flow into the inductor L, and at this moment,the back electromotive force in the direction of hindering the flow ofthe charge occurs in the inductor L. Subsequently, when the currentflowing through the inductor L reaches the peak and then takes adownward turn, the electromotive force in the direction (the directionof urging the current to continue to flow) of preventing the currentfrom decreasing occurs in turn in the inductor L. As a result, thereoccurs the state (the state shown in FIG. 3B) in which all the positivecharge once located on the lower surface side of the piezoelectricelement 108 c has been migrated to the upper surface side. The positivecharge having returned to the upper surface side of the piezoelectricelement 108 c in this way is migrated again to the lower surface side insuch a manner as described above with reference to FIGS. 3B through 3F.

As described above, if the switch SW is set to the ON state in the statein which the charge is stored in the capacitor Cg and is then kept inthe ON state, there occurs a kind of resonant phenomenon in which thedirection of the current is reversed alternately between thepiezoelectric element 108 c and the inductor L. The period of theresonant phenomenon corresponds to the resonance period T of theso-called LC resonant circuit, and is therefore obtained by the formulaT=2π(LC)^(0.5), assuming that the value (capacitance) of the capacitivecomponent Cg included in the piezoelectric element 108 c is C, and thevalue (inductance) of the inductive component of the inductor L is L.Therefore, the time immediately after (the state shown in FIG. 3C)setting the switch SW to the ON state until the state shown in FIG. 3Foccurs is obtained as T/2.

Therefore, the switch SW is set to the OFF state as shown in FIG. 4A atthe time point when T/2 has elapsed after setting the switch SW to theON state. Then, the piezoelectric element 108 c (the beam 104, to beprecise) is in turn deformed downward (so that the lower surface sidehas a concave shape) in the present state. Although the piezoelectricelement 108 c is deformed upward in FIG. 3A described above, since thepiezoelectric element 108 c is deformed downward in FIG. 4A, the currentflows from the current source in the negative direction, and the chargeis stored in the capacitor Cg so that the voltage between the terminalsof the piezoelectric element 108 c increases toward the negative side.Since the positive charge is distributed on the lower surface side ofthe piezoelectric element 108 c and the negative charge is distributedon the upper surface side thereof in the stage prior to deforming thepiezoelectric element 108 c (the beam 104, to be precise) downward asdescribed above with reference to FIGS. 3A through 3F, it results that anew positive charge is stored on the lower surface side and a newnegative charge is stored on the upper surface side in addition to thesecharges. FIG. 4B shows the state in which the new charges are stored inthe piezoelectric element 108 c by deforming the piezoelectric element108 c (the beam 104, to be precise) in the state of setting the switchSW to the OFF state.

When setting the switch SW to the ON state in this state, the positivecharge stored on the lower surface side of the piezoelectric element 108c is urged to flow into the inductor L. At this moment, since the backelectromotive force occurs in the inductor L (see FIG. 4C), the currentstarts flowing gradually, and eventually reaches the peak, and thenmakes a downward turn. Then, the electromotive force occurs in theinductor L in the direction (the direction of urging the current tocontinue to flow) of preventing the current from decreasing (see FIG.4E), and the current continues to flow due to the electromotive force.Eventually, there occurs the state in which all the positive charge oncedistributed on the lower surface side of the piezoelectric element 108 chas been migrated to the upper surface side, and all the negative chargeonce distributed on the upper surface side thereof has been migrated tothe lower surface side (see FIG. 4F). The time necessary for migratingall the positive charge on the lower surface side to the upper surfaceside and migrating all the negative charge on the upper surface side tothe lower surface side is equal to the time T/2 corresponding to a halfof the resonance period T of the LC resonant circuit. Therefore, bysetting the switch SW to the OFF state when the time T/2 has elapsedafter setting the switch SW to the ON state, and then deforming in turnthe piezoelectric element 108 c (the beam 104, to be precise) upward (sothat the upper surface side has a concave shape), the positive andnegative charges can further be stored in the piezoelectric element 108c.

As explained hereinabove, in the power generation unit 100 according tothe present embodiment, by deforming the piezoelectric element 108 c tothereby generate the charge, and then connecting the piezoelectricelement 108 c to the inductor L to thereby form the resonant circuit fora half cycle of the resonance period T, the distributions of thepositive and negative charges in the piezoelectric element 108 c arereversed. Subsequently, the piezoelectric element 108 c is in turndeformed in the opposite direction to thereby generate new charges.Since the distributions of the positive and negative charges in thepiezoelectric element 108 c have been reversed, it results that thecharges newly generated are stored in the piezoelectric element 108 c.Subsequently, the piezoelectric element 108 c is connected again to theinductor L for a half cycle of the resonance period T to thereby reversethe distributions of the positive and negative charges in thepiezoelectric element 108 c, and then the piezoelectric element 108 c isdeformed in the opposite direction. By repeating such operations, it ispossible to increase the charge stored in the piezoelectric element 108c every time the piezoelectric element 108 c is deformed in a repeatedmanner.

As described above with reference to FIGS. 2A through 2D, in the powergeneration unit 100 according to the present embodiment, the peculiarphenomenon of shifting the waveform of the voltage generated between thefirst electrode 108 a and the second electrode 108 b occurs every timethe switch SW is set to the ON state, and this phenomenon occurs due tothe following mechanism. That is, in the period A shown in FIG. 2D, forexample, although the voltage is generated between the first electrode108 a and the second electrode 108 b in accordance with the deformationof the piezoelectric element 108 c (the beam 104, to be precise), sincethe first electrode 108 a and the second electrode 108 b are connectedto the rectifier 120, the charge corresponding to the part exceeding thevoltage of the sum of VC1 and 2 Vf flows into the capacitor C1 connectedto the rectifier 120. As a result, when setting the switch SW to the ONstate at the time point when the deformation of the beam 104 reaches thepeak, the positive and negative charges remaining in the piezoelectricelement 108 c at that moment are migrated via the inductor L, and thelocations of the positive and negative charges in the piezoelectricelement 108 c are replaced with each other.

When deforming the beam 104 in the opposite direction in the state inwhich the locations of the positive and negative charges are replacedwith each other, the voltage waveform due to the piezoelectric effectappears between the first electrode 108 a and the second electrode 108 bof the piezoelectric element 108 c. In other words, it results that thevoltage variation due to the deformation of the piezoelectric element108 c occurs in the state in which the polarities of the first electrode108 a and the second electrode 108 b of the piezoelectric element 108 care replaced with each other. As a result, there appears in the period Bshown in FIG. 2D the voltage waveform looked as if the voltage waveformgenerated in the piezoelectric element 108 c due to the deformation ofthe beam 104 is shifted. However, as described above, since the chargecorresponding to the part exceeding the voltage of the sum of VC1 and 2Vf flows into the capacitor C1, the voltage between the first electrode108 a and the second electrode 108 b of the piezoelectric element 108 cis clipped at the voltage of the sum of VC1 and 2 Vf. Subsequently, whensetting the switch SW to the ON state for the time half as long as theresonance period T, the locations of the positive and negative chargesremaining in the piezoelectric element 108 c are replaced with eachother. By the deformation of the beam 104 starting from that state, thevoltage waveform due to the piezoelectric effect appears in thepiezoelectric element 108 c. Therefore, it results that there appearsalso in the period C shown in FIG. 2D the voltage waveform looked as ifthe voltage waveform due to the deformation of the beam 104 is shifted.

As described above with reference to FIGS. 2A through 2D, in the powergeneration unit 100 according to the present embodiment, there alsooccurs the phenomenon that the shift amount of the voltage waveformgradually increases while the beam 104 repeats the deformation.Therefore, there can be obtained a significant advantage that thevoltage higher than the electrical potential difference caused betweenthe first electrode 108 a and the second electrode 108 b due to thepiezoelectric effect of the piezoelectric element 108 c can be stored inthe capacitor C1. Such a phenomenon is caused by the followingmechanism.

Firstly, as shown in the period A or the period B in FIG. 2D, in thecase in which the capacitor C1 has not been charged, since the chargeflows from the piezoelectric element 108 c into the capacitor C1 whenthe voltage generated between the terminals of the piezoelectric element108 c exceeds 2 Vf of the rectifier 120, the voltage appearing betweenthe first electrode 108 a and the second electrode 108 b is clipped at 2Vf. However, as the charges are stored in the capacitor C1 in such amanner as described above, the voltage between the terminals of thecapacitor C1 increases. Then, thereafter, the charge does not flow intothe capacitor C1 from the piezoelectric element 108 c until the voltagebetween the first electrode 108 a and the second electrode 108 b reachesa voltage higher than the sum of VC1 and 2 Vf. Therefore, the value atwhich the voltage between the first electrode 108 a and the secondelectrode 108 b is clipped rises gradually as the charges are stored inthe capacitor C1.

In addition, as described above with reference to FIGS. 3A through 3Fand 4A through 4F, on the condition that the charge is prevented fromflowing out from the piezoelectric element 108 c, the charges in thepiezoelectric element 108 c continue to increase every time thepiezoelectric element 108 c (the beam 104, to be precise) is deformed,and the voltage between the first electrode 108 a and the secondelectrode 108 b rises in conjunction therewith. Therefore, if the lossof the charge when flowing through the inductor L and the switch SW isnot considered, it is possible to increase the voltage between the firstelectrode 108 a and the second electrode 108 b. Therefore, according tothe power generation unit 100 of the present embodiment, it becomespossible to generate the electrical power in the condition in which thevoltage is automatically raised to the voltage necessary to drive theelectrical load without providing an additional step-up circuit.

A-4. Switching Timing of Switch

As explained hereinabove, in the power generation unit 100 according tothe present embodiment, by applying the cyclic deformation to thepiezoelectric element 108 c (the beam 104, to be precise), andconnecting the piezoelectric element 108 c to the inductor L for aperiod of time half as long as the resonance period T at the timing atwhich the deformation direction is switched, it is possible to obtain anexcellent feature that the charge can efficiently be stored in thecapacitor C1, and in addition, miniaturization can easily be achievedbecause no step-up circuit is required. However, due to thecircumstances of the operation speed of the control section 130 and theswitch SW, the timing at which the control section 130 sets the switchSW to the ON state does not necessarily coincide completely with thetiming at which the deformation direction of the beam 104 is switched.However, it is possible to step-up the voltage Vgen generated betweenthe first electrode 108 a and the second electrode 108 b by setting theswitch SW to the ON state for the period of time half as long as theresonance period T of the LC resonant circuit with a period coincidingwith the characteristic vibration period of the beam 104 even if thetiming at which the switch SW is set to the ON state does not completelycoincide with the timing at which the deformation direction of the beam104 is switched. Hereinafter, the reason therefore will be explained.

FIG. 5A shows the state of the voltage Vgen generated between the firstelectrode 108 a and the second electrode 108 b if the switch SW is notset to the OFF state after setting the switch SW to the ON state at thetime point t1 at which the deformation direction of the beam 104 isswitched. FIG. 5B is a diagram obtained by enlarging a part of thewaveform shown in FIG. 5A on and after the time point t1. It is assumedin the example shown in FIGS. 5A and 5B that the rectifier 120 and thecapacitor C1 are eliminated.

At the time point t1, the voltage Vgen has a peak, and by setting theswitch SW to the ON state, the voltage Vgen is attenuated whilealternately showing positive and negative peak values Vp1, Vp2, Vp3,Vp4, Vp5, Vp6, . . . with a period (the time points t1, t2, t3, t4, t5,t6, . . . ) half as long as the resonance period T of the LC resonantcircuit. If the switch SW is set to the OFF state at the time point t2when T/2 has elapsed from the time point t1, the shift amount of thevoltage Vgen described above is obtained as the sum (|Vp1|+|Vp2|) of theabsolute value of Vp1 and the absolute value of Vp2. As explained withreference to FIGS. 3A through 3F, and 4A through 4F, since Vp2 is avoltage value when the positive and negative charges of the capacitivecomponent Cg are replaced with each other by resonance of the LCresonance circuit, the larger the absolute value of Vp1 is, the largerthe absolute value of Vp2 becomes. Therefore, the larger the absolutevalue of Vp1 is, the larger the shift amount of the voltage Vgenbecomes.

FIG. 6 shows the state of the voltage Vgen generated between the firstelectrode 108 a and the second electrode 108 b in the case in which theswitch SW is set to the ON state only for the period of T/2 every timethe deformation direction of the beam 104 is switched. It is assumed inalso the example shown in FIG. 6 that the rectifier 120 and thecapacitor C1 are eliminated. Assuming that the amplitude of the voltageVpzt due to the electromotive force generated by the piezoelectricelement 108 c is constant, if the switch SW is set to the ON state forthe period of T/2 at the timing at which the voltage Vgen first reachesthe voltage value V₁ as a positive peak value, the voltage Vgen isshifted V₁+Va toward the negative side. Then, the voltage value V₂ ofVgen when the second time the switch SW is set to the ON state isV₂=−(Va+2V₁), and if the switch SW is set to the ON state for the periodof T/2, the voltage Vgen is shifted Vb+Va+2V₁ toward the positive side.Similarly, the voltage value V₃ of Vgen when the third time the switchSW is set to the ON state is V₃=Vb+2V₁, and if the switch SW is set tothe ON state for the period of T/2, the voltage Vgen is shiftedVc+Vb+2V₁ toward the negative side. Similarly, the voltage value V₄ ofVgen when the fourth time the switch SW is set to the ON state isV₄=−(Vc+2V₁), and if the switch SW is set to the ON state for the periodof T/2, the voltage Vgen is shifted Vd+Vc+2V₁ toward the positive side.Similarly, the voltage value V₅ of Vgen when the fifth time the switchSW is set to the ON state is V₅=Vd+2V₁. Here, since the voltage value V₂is obtained as V₂=−(Va+2V₁), |V₂|>|V₁| is obviously true. Since thereference symbols V₁, V₂ denote the voltage values corresponding to thevoltage value Vp1 shown in FIG. 5B, and the reference symbols Va, Vbdenote the voltage values corresponding to the voltage value Vp2 shownin FIG. 5B, and |V₂|>|V₁| is true, Vb>Va is necessarily fulfilled.Therefore, since V₂ is obtained as V₂=−(Va+2V₁), V₃ is obtained asV₃=Vb+2V₁, and Vb>Va is true, |V₃|>|V₂| is true. Similarly, since|V₃|>|V₂| is true, Vc>Vb is necessarily fulfilled, and since V₃=Vb+2V₁,V₄=−(Vc+2V₁) are obtained, and Vc>Vb is true, |V₄|>|V₃| is true.Similarly, since |V₄|>|V₃| is true, Vd>Vc is necessarily fulfilled, andsince V₄=−(Vc+2V₁), V₅=Vd+2V₁ are obtained, and Vd>Vc is true, |V₅|>|V₄|is true. In essence, by setting the switch SW to the ON state for theperiod of T/2 at the timing at which the deformation direction of thebeam 104 is switched, the absolute value of the voltage Vgen generatedbetween the first electrode 108 a and the second electrode 108 b isstepped up in such a manner as |V₁|<|V₂|<|V₃|<|V₄|<|V₅|< . . . .

The same can be applied to the case in which the timing of switching thedeformation direction of the beam 104 and the timing of setting theswitch SW to the ON state are shifted from each other. FIG. 7A shows thestate of the voltage Vgen generated between the first electrode 108 aand the second electrode 108 b in the case in which the switch SW is setto the ON state for the period of T/2 after the timing at which thedeformation direction of the beam 104 is switched, and FIG. 7B shows thestate of the voltage Vgen generated between the first electrode 108 aand the second electrode 108 b in the case in which the switch SW is setto the ON state for the period of T/2 prior to the timing at which thedeformation direction of the beam 104 is switched. It is assumed in alsothe examples shown in FIGS. 7A and 7B that the rectifier 120 and thecapacitor C1 are eliminated.

In the examples shown in FIGS. 7A and 7B, similarly to the example shownin FIG. 6, the voltage Vgen takes the voltage value V₂=−(Va+2V₁) whenthe second time the switch SW is set to the ON state with respect to thevoltage value V₁ when the switch SW is first set to the ON state, thevoltage value V₃=Vb+2V₁ when the third time the switch SW is set to theON state, the voltage value V₄=−(Vc+2V₁) when the fourth time the switchSW is set to the ON state, the voltage value V₅=Vd+2V₁ when the fifthtime the switch SW is set to the ON state, and so on. Here, since thevoltage values V₂, V₃, V₄, V₅, . . . are expressed by the same formulasas those of the voltage values V₂, V₃, V₄, V₅, . . . in the case of FIG.6, the conditions V₂>V₁, V₃>V₂, V₄>V₃, V₅>V₄, . . . are also fulfilled.Therefore, also in the case in which the switch SW is set to the ONstate for the period of T/2 at the timing shifted before or after thetiming at which the deformation direction of the beam 104 is switched,the voltage Vgen is stepped up in such a manner as|V₁|<|V₂|<|V₃|<|V₄|<|V₅|< . . . . It should be noted that since thehigher the voltage value V₁ is, the larger the voltage values Va, Vb,Vc, Vd, . . . become, the rate of stepping up the voltage Vgen is higherand the power generation efficiency is higher in the example shown inFIG. 6 than in the examples shown in FIGS. 7A and 7B.

In the case (the case of V₁=0 in FIGS. 7A and 7B) in which the switch SWis set to the ON state for the period of T/2 at the timing at which thedisplacement of the beam 104 vanishes (the voltage Vgen reaches 0), theresonance of the LC resonant circuit fails to occur, and the voltageVgen does not rise.

As explained hereinabove, even if the switch SW is set to the ON stateat an arbitrary timing (it should be noted that the timing at which thedisplacement of the beam 104 vanishes (the voltage Vgen is equal to 0)is excepted), the voltage generated between the first electrode 108 aand the second electrode 108 b can be stepped up by setting the switchSW to the ON state for the period half as long as the resonance period Tof the LC resonant circuit.

Although it is preferable to set the switch SW to the ON state only forthe period half as long as the resonance period T of the LC resonantcircuit in order to improve the power generation efficiency, it ispossible to at least step up the voltage Vgen generated between thefirst electrode 108 a and the second electrode 108 b even by setting theswitch SW to the ON state for a predetermined period of time. Forexample, FIG. 8 shows an example of the voltage Vgen generated betweenthe first electrode 108 a and the second electrode 108 b in the case inwhich the switch SW is set to the ON state only for the periodthree-halves times as long as the resonance period T at the timing atwhich the deformation direction of the beam 104 is switched. In essence,the case corresponds to the case in which the switch SW is set to the ONstate at the time point t1 shown in FIG. 5B, and is then set to the OFFstate at the time point t3. It is assumed in also the example shown inFIG. 8 that the rectifier 120 and the capacitor C1 are eliminated.

In the example shown in FIG. 8, similarly to the example shown in FIG.6, the voltage Vgen takes the voltage value V₂=−(Va+2V₁) when the secondtime the switch SW is set to the ON state with respect to the voltagevalue V₁ when the switch SW is first set to the ON state, the voltagevalue V₃=Vb+2V₁ when the third time the switch SW is set to the ONstate, the voltage value V₄=−(Vc+2V₁) when the fourth time the switch SWis set to the ON state, the voltage value V₅=Vd+2V₁ when the fifth timethe switch SW is set to the ON state, and so on, and the voltage Vgen isstepped up in such a manner as |V₁|<|V₂|V₃|<|V₄|<|V₅|< . . . . It shouldbe noted that since the higher the voltage value V₁ is, the larger thevoltage values Va, Vb, Vc, Vd, . . . become, the rate of stepping up thevoltage Vgen is higher and the power generation efficiency is higher inthe example shown in FIG. 6 than in the example shown in FIG. 8.

On the other hand, FIG. 9 shows the state of the voltage Vgen generatedbetween the first electrode 108 a and the second electrode 108 b in thecase in which the switch SW is set to the ON state only for a period aquarter as long as the resonance period T at the timing at which thedeformation direction of the beam 104 is switched. In essence, the casecorresponds to the case in which the switch SW is set to the ON state atthe time point t1 shown in FIG. 5B, and is then set to the OFF state atthe time point (t1+t2)/2. It is assumed in also the example shown inFIG. 9 that the rectifier 120 and the capacitor C1 are eliminated.

In the example shown in FIG. 9, the voltage Vgen takes the voltage valueV₂=−2V₁ when the second time the switch SW1 is set to the ON state withrespect to the voltage value V₁ when the switch SW is first set to theON state, the voltage value V₃=2V₁ when the third time the switch SW isset to the ON state, the voltage value V₄=−2V₁ when the fourth time theswitch SW is set to the ON state, the voltage value V₅=2V₁ when thefifth time the switch SW is set to the ON state, and so on. In otherwards, the voltage Vgen can be stepped up to 2V₁, but is not stepped upto a level exceeding 2V₁.

Similarly, also in the case of setting the switch SW to the ON stateonly for the period obtained by multiplying either one of 3/4, 5/4, 7/4,9/4, . . . by the resonance period T at the timing at which thedeformation direction of the beam 104 is switched, the voltage values ofV₂=−2V₁, V₃=2V₁, V₄=−2V₁, V₅=2V₁, . . . are obtained, and the voltageVgen can be stepped up to 2V₁, but is not stepped up beyond 2V₁.

According to the phenomenon described above, due to the resonance of theLC resonant circuit, by setting the switch SW to the OFF state at leastwhen the polarity of the voltage Vgen is changed to the oppositepolarity to the polarity thereof at the time point when the switch SW isset to the ON state, the voltage Vgen continues to rise. In essence, itis possible to efficiently step up the voltage Vgen by setting thepredetermined period during which the switch SW is set to the ON stateto the period at least longer than (n+1/4)T and shorter than (n+3/4)T (ndenotes an arbitrary integer equal to or greater than 0) with respect tothe resonance period T of the LC resonant circuit.

As described above, in the case of setting the switch SW to the ON stateonly for the period half as long as the resonance period T of the LCresonant circuit, the shift amount when switching the switch SW takesthe maximum value, and therefore, the highest power generationefficiency can be obtained. Therefore, in the power generation unit 100according to the present embodiment, the control section 130 sets theswitch SW to the ON state with the period coinciding with thecharacteristic vibration period of the beam 104, and sets the switch SWto the OFF state when the time half as long as the resonance period T ofthe LC resonant circuit has elapsed.

However, it is not so easy to set the switch SW to the ON state at thetiming at which the deformation direction of the beam 104 is switched.For example, assuming that the magnitude of the displacement of the beam104 reaches the maximum value at the timing at which the deformationdirection of the beam 104 is switched, it is also possible to adopt theconfiguration of using a mechanical contact which is set to the ON stateat the timing at which the beam 104 takes the maximum displacement.However, if the adjustment of the contact fails, it results that theefficiency is dramatically degraded. Therefore, in the power generationunit 100 according to the present embodiment, the current Ipzt2generated in the piezoelectric device 110 is detected to thereby set theswitch SW to the ON state. The timing at which the deformation directionof the piezoelectric device 110 is switched coincides with the timing(the timing at which the current value reaches 0) at which the directionof the current Ipzt2 due to the charge generated by the piezoelectricdevice 110 is switched. Therefore, by detecting the current Ipzt2generated in the piezoelectric device 110, the switch SW can easily beset to the ON state (a conductive state) at the timing at which thedeformation direction of the beam 104 (the deforming member) isswitched.

FIGS. 10A through 10C are explanatory diagrams showing the reason thatthe switch SW can be controlled at an appropriate timing by detecting acurrent Ipzt2 generated in the controlling piezoelectric device 110.FIG. 10A shows the displacement of the beam 104. FIG. 10B shows how thecurrent Ipzt2 generated in the piezoelectric device 110 varies due tothe vibration of the beam 104. FIG. 10C shows the ON/OFF state of theswitch SW.

As described above with reference to FIGS. 3A through 3F, 4A through 4F,5A, 5B, 6, 7A, 7B, 8, and 9, the electrical power can be generated withthe highest efficiency in the case of setting the switch SW to the ONstate at the timing at which the displacement u of the beam 104 reachesthe extreme value. As is obvious from the comparison between FIGS. 10Aand 10B, the timing at which the displacement u of the beam 104 takesthe extreme value coincides with the timing at which the current Ipzt2generated in the piezoelectric device 110 vanishes. The reason thereforeis that since the piezoelectric device 110 is not connected to theinductor L or the capacitor C1, the variation in the charge is directlyreflected on the variation in the current Ipzt2 generated in thepiezoelectric device 110.

Therefore, as indicated by the outlined arrow in FIG. 10B, by detectingthe timing at which the current Ipzt2 generated in the piezoelectricdevice 110 reaches 0, and then setting the switch SW to the ON state foronly a predetermined period (e.g., the period (T/2) half as long as theresonance period T of the LC resonant circuit described above) startingfrom that timing, it becomes possible to efficiently generate theelectrical power.

FIG. 11 is a flowchart for explaining a switch control process as anexample of a method of controlling the power generation unit 100according to the present embodiment. The method of controlling the powergeneration unit 100 according to the present embodiment includes thestep of detecting the current generated in the piezoelectric device 110,and the step of electrically connecting the piezoelectric device 108 andthe inductor L to each other via the switch SW based on the detectionresult of the current.

In the switch control process shown in FIG. 11, firstly, the controlsection 130 detects (step S100) the current generated in thepiezoelectric device 110. In the present embodiment, the current detectcircuit 134 of the control section 130 detects the current flowing inthe capacitor 132 to thereby detect the current generated in thepiezoelectric device 110. Since the capacitor 132 is connected inparallel to the piezoelectric device 110 as shown in FIG. 1B, thecurrent having the same phase as that of the current generated in thepiezoelectric device 110 flows in the capacitor 132. Therefore, bydetecting the current flowing in the capacitor 132, the timing (thetiming at which the current vanishes) at which the direction of thecurrent due to the charge generated in the piezoelectric device 110 isswitched can easily be detected.

FIG. 12 is a block diagram showing an example of a configuration of thecurrent detect circuit 134.

As a current detector 1341, a device known to the public such as a Hallelement current sensor or a shunt resistance can be used.

An amplifier 1342 amplifies an output signal (Id) of the currentdetector 1341 at a predetermined gain. An absolute-value circuit 1343outputs an absolute value signal of an output signal (Idamp) of theamplifier 1342. The amplifier 1342 and the absolute-value circuit 1343are not essential circuits, but are added for making it easy for thecomparator 1344 to detect presence or absence of the current.

The comparator 1344 binarizes the output signal (Jabs) of theabsolute-value circuit 1343 (converts the output signal into pulses),and then outputs the result. At the timing of the falling edge of theoutput signal (Ipls) of the comparator 1344, the current flowing in thecapacitor 132 vanishes. It is also possible to arrange that the state inwhich a little current flows is detected instead of the state in whichno current flows at all. This configuration is adopted for preventingmalfunction of the comparator 1344 due to noises when no current flows.If a lot of margin is taken here, the power generation efficiency isdegraded due to the shift of the detection timing, and therefore, it ispreferable to reduce the noise as much as possible, and perform thedetection at the timing at which the current approximates 0.

In the switch control process shown in FIG. 11, after the step S100,whether or not the current value detected in the step S100 has made zerocrossing is determined (step S102). In the present embodiment, thecontrol circuit 136 performs the determination in the step S102 based onthe output signal (Ipls) of the current detect circuit 134 of thecontrol section 130. In the case (the case in which NO is determined inthe step S102) in which zero crossing has not been made, the step S100and the step S102 are repeated.

If the current value makes the zero crossing (the case of YES in thestep S102), the control section 130 switches (step S104) the switch SWto the ON state. In the present embodiment, the control circuit 136 ofthe control section 130 outputs a control signal to the switch SW tothereby switch the switch SW to the ON state.

After the step S104, the control section 130 starts (step S106) a timer.In the present embodiment, it is possible for the control circuit 136 ofthe control section 130 to have the timer.

After the step S106, the control section 130 determines (step S108)whether or not the period (T/2) half as long as the resonance period Tof the resonant circuit composed of the capacitance component Cg of thepiezoelectric device 108 and the inductor L. In the present embodiment,the control circuit 136 of the control section 130 performs thedetermination in the step S108. If the control circuit 136 determines(the case of NO in the step S108) that the time T/2 has not elapsed, thestep S108 is repeated.

If the control circuit 136 determines (the case of YES in the step S108)that the time T/2 has elapsed, the control section 130 switches (stepS110) the switch SW to the OFF state. In the present embodiment, thecontrol circuit 136 of the control section 130 outputs a control signalto the switch SW to thereby switch the switch SW to the OFF state. Afterthe step S110, the control section 130 repeats the steps S100 throughS110.

By switching between the ON/OFF states of the switch SW in such a manneras described hereinabove, the switch SW can be switched between theON/OFF states at appropriate timings in accordance with the movement ofthe beam 104, and therefore, it becomes possible to efficiently generatethe electrical power using the power generation unit 100.

Since the switch SW is switched between the ON/OFF states based on thecurrent generated in the piezoelectric device 110, the timing can bedetermined based on whether or not the current value traverses thereference value instead of whether or not the extreme value of thecurrent value is reached. Therefore, it is possible to accuratelydetermine the timing for switching the switch SW. Thus, the powergeneration efficiency can be improved.

B. SECOND EMBODIMENT

In the explanation of the power generation unit 100 according to thefirst embodiment described above, it is assumed that a singlecontrolling piezoelectric device 110 is disposed. However, it is notnecessarily required to provide the single controlling piezoelectricdevice 110, but a plurality of such controlling piezoelectric devicescan also be provided. Hereinafter, a second embodiment with such aconfiguration will be explained. The constituents substantially the sameas those of the first embodiment will also be attached with the samereference numerals in the second embodiment, and the detailedexplanation therefore will be omitted.

FIGS. 13A through 13C are explanatory diagrams showing the powergeneration unit 100 according to the second embodiment provided with theplurality of controlling piezoelectric devices. FIG. 13A is a plan viewthereof viewed from one surface of the beam 104. FIG. 13B is a plan viewthereof viewed from the other surface of the beam 104. FIG. 13A showsthe power-generating piezoelectric device 108 disposed on the onesurface of the beam 104, and FIG. 13B shows two controllingpiezoelectric devices (the piezoelectric device 110 and thepiezoelectric device 114) disposed on the other surface of the beam 104.As is obvious from the comparison between FIGS. 13A and 13B, thecontrolling piezoelectric devices 110, 114 have a length (a length inthe longitudinal direction of the beam 104) the same as that of thepower-generating piezoelectric device 108, but have a width (a length inthe direction along the shorter dimension of the beam 104) narrower thana half of that of the power-generating piezoelectric device 108. The twocontrolling piezoelectric devices 110, 114 are disposed at positionsshifted toward both sides of the beam 104 in the width directionthereof. The piezoelectric device 114 is configured including apiezoelectric element 114 c formed of a piezoelectric material such aslead zirconium titanate (PZT), and a first electrode (an upperelectrode) 114 a and a second electrode (a lower electrode) 114 b eachformed of a metal thin film on the surfaces of the piezoelectric element114 c. The first electrode (the upper electrode) 114 a and the secondelectrode (the lower electrode) 114 b are disposed so as to be opposedto each other across the piezoelectric element 114 c.

If the power-generating piezoelectric device 108 has the maximuminstallable length and width with respect to the beam 104, the amount ofpower generation of the power-generating piezoelectric device 108becomes large, and if the controlling piezoelectric devices 110, 114have the minimum installable width (length in the direction along theshorter dimension of the beam 104), the displacement resistance of thebeam 104 due to the controlling piezoelectric devices 110, 114 isreduced, and therefore, high power generation efficiency is obtained.

By disposing the two controlling piezoelectric devices 110, 114 at thepositions shifted toward the both sides of the beam 104 in the widthdirection thereof, the controlling piezoelectric devices 110, 114 canset the switch SW to the ON/OFF states at appropriate timings even inthe case in which the beam 104 generates the displacement differentbetween the vertical and horizontal positions, and therefore, the powergeneration unit 100 can be used in a variety of situations.

FIG. 13C shows a circuit diagram of the power generation unit 100according to the second embodiment provided with the two controllingpiezoelectric devices 110, 114. The first controlling piezoelectricdevice 110 is expressed as a combination of a current source and acapacitor (a capacitance component) Cs1 for storing charges, and thesecond controlling piezoelectric device 114 is expressed as acombination of a current source and a capacitor (a capacitive component)Cs2 for storing charges. The first electrode 110 a and the secondelectrode 110 b of the first controlling piezoelectric device 110 areconnected to the control section 130, and the first electrode 114 a andthe second electrode 114 b of the second controlling piezoelectricdevice 114 are also connected to the control section 130.

In the example shown in FIG. 13C, the control section 130 is configuredincluding the capacitor 132 connected in parallel to the piezoelectricdevice 110, the current detect circuit 134 for detecting the currentflowing to the capacitor 132, a capacitor 133 connected in parallel tothe piezoelectric device 114, a current detect circuit 135 for detectingthe current flowing to the capacitor 133, and a control circuit 136 forcontrolling the switch SW based on the current detected at least eitherone of the current detect circuit 134 and the current detect circuit135. The current detect circuit 135 can be configured similarly to thecurrent detect circuit 134.

The control section 130 selects either one of a pair of first electrode110 a and the second electrode 110 b and a pair of first electrode 114 aand the second electrode 114 b, and then detects the current generatedin the selected one of the piezoelectric device 110 and thepiezoelectric device 114 to thereby set the switch SW to the conductivestate for a predetermined period of time. For example, the amount ofpower generation has previously been measured when installing the powergeneration unit 100 in the case of generating the electrical power whiledetecting the current generated in the piezoelectric device 110 and inthe case of generating the electrical power while detecting the currentgenerated in the piezoelectric device 114. Either one of the cases witha larger amount of power generation has previously been selected using aswitch (not shown) or the like provided to the control section 130. Ifeither one of the piezoelectric device 110 and the piezoelectric device114 has previously been selected in such a manner as described above, byperforming the switch control process described above with reference toFIG. 11, the ON/OFF control of the switch SW can be performed.

Although the two piezoelectric devices 110, 114 generate broadly similarcurrent waveforms, a slight difference in current waveform and magnitudeof the current amplitude can occur due to the factors such as thestructure of the beam 104 and the production tolerance. Further, if thedifference occurs in the current waveform, there is a possibility ofcausing the difference in power generation amount, and if the differenceoccurs in the magnitude of the current amplitude, there is a possibilityof controlling the switch SW at a more appropriate timing when using oneof the piezoelectric devices (having higher sensitivity as a sensor)with which a larger current amplitude can be obtained. Therefore, byhaving previously measured the power generation amount in the case ofgenerating the electrical power while detecting the current generated inthe piezoelectric device 110 and in the case of generating theelectrical power while detecting the current generated in thepiezoelectric device 114, and then selecting one with larger powergeneration amount, it becomes possible to more efficiently generate theelectrical power.

C. THIRD EMBODIMENT

FIG. 14 is a circuit diagram showing the electrical structure of thepower generation unit 100 according to a third embodiment. Themechanical structure of the power generation unit 100 according to thethird embodiment is the same as the structure shown in FIG. 1A. Theconstituents substantially the same as those of the first embodimentwill also be attached with the same reference numerals in the thirdembodiment, and the detailed explanation therefore will be omitted.

In the power generation unit 100 according to the third embodiment, thecontrolling piezoelectric device 110 is also provided in addition to thepower-generating piezoelectric device 108, and the voltage generated inthe piezoelectric device 110 is detected to thereby control the switchSW.

The power generation unit 100 according to the third embodiment includesa control section 130 a. The control section 130 a performs the ON/OFFcontrol of the switch SW. Specifically, the control section 130 adetects the voltage generated in the piezoelectric device 110, and ifthe voltage detected has a value equal to or higher than a predeterminedvalue, the control section 130 a sets the switch SW to the conductivestate to thereby electrically connect the piezoelectric device 108 andthe inductor L to each other via the switch SW. In the presentembodiment, the control section 130 a is configured including a voltagedetect circuit 138 for detecting the voltage generated in thepiezoelectric device 110 and a control circuit 136 a for controlling theswitch SW based on the voltage detected by the voltage detect circuit138. The control circuit 136 a can also be formed of a centralprocessing unit (CPU). Details of the operation of the control section130 a will be described later.

FIGS. 15A through 15C are explanatory diagrams showing the reason thatthe switch SW can be controlled at an appropriate timing by detecting avoltage generated in the controlling piezoelectric device 110. FIG. 15Ashows the displacement of the beam 104. FIG. 15B shows how theelectromotive force Vpzt2 generated in the piezoelectric device 110varies due to the vibration of the beam 104. FIG. 15C shows the ON/OFFstate of the switch SW.

As described above with reference to FIGS. 3A through 3F, and 4A through4F, the electrical power can be generated with the highest efficiency inthe case of setting the switch SW to the ON state at the timing at whichthe displacement u of the beam 104 reaches the extreme value. As isobvious from the comparison between FIGS. 15A and 15B, the timing atwhich the displacement u of the beam 104 takes the extreme valuecoincides with the timing at which the electromotive force Vpzt2 of thepiezoelectric device 110 has an extreme value. The reason therefore isas follows. Firstly, even if the charge is generated due to thedeformation of the piezoelectric device 108, the electromotive forceVpzt of the piezoelectric device 108 fails to completely coincide withthe displacement of the beam 104 in consequence of the phenomenon thatthe charge is pulled out by the inductor L or the phenomenon that thecharge flows to the output capacitor C1. In contrast, the piezoelectricdevice 110 is not connected to the inductor L or the output capacitorC1, the variation in the charge is directly reflected on the variationin the electromotive force Vpzt2 of the piezoelectric device 110.Therefore, the timing at which the electromotive force Vpzt2 of thepiezoelectric device 110 takes an extreme value coincides with thetiming at which the displacement u of the beam 104 takes the extremevalue.

Therefore, as indicated by the arrows in FIG. 15B, by detecting thetiming at which the electromotive force Vpzt2 of the piezoelectricdevice 110 takes the extreme values, and then setting the switch SW tothe ON state for the period (T/2) half as long as the resonance perioddescribed above starting from that timing, it becomes possible toefficiently generate the electrical power.

FIG. 16 is a flowchart for explaining a switch control process as anexample of a method of controlling the power generation unit 100according to the present embodiment. The method of controlling the powergeneration unit 100 according to the present embodiment includes thestep of detecting the voltage generated in the piezoelectric device 110,and the step of electrically connecting the piezoelectric device 108 andthe inductor L to each other via the switch SW based on the detectionresult of the voltage.

When starting the switch control process, firstly, the voltage detectcircuit 138 detects (step S200) the voltage Vpzt2 generated in thepiezoelectric device 110. Then, the control circuit 136 a determines(step S202) whether or not the voltage value thus detected in thevoltage detect circuit 138 reaches a peak (i.e., whether or not thevoltage value reaches the extreme value). Whether or not the voltagevalue reaches the peak can be determined in such a manner that, forexample, it can be determined that the voltage value reaches the peak ifthe sign of the differential value obtained by performing thedifferentiation of the voltage waveform is varied.

Alternatively, since it is conceivable that the amplitude of thedisplacement of the beam 104 is roughly constant, it is conceivable thatthe voltage generated in the controlling piezoelectric device 110 isalso roughly constant. Therefore, the maximum voltage value Vmax and theminimum voltage value Vmin have previously been stored, and then thevoltage generated in the piezoelectric device 110 is compared with themaximum voltage value Vmax and the minimum voltage value Vmin. It isalso possible to determine that the voltage value reaches the peak ifthe generated voltage of the piezoelectric device 110 exceeds themaximum voltage value Vmax, or falls below the minimum voltage valueVmin. The beam 104 is not necessarily deformed with completely the sameamplitude, and therefore, the amplitude of the voltage generated in thepiezoelectric device 110 does not necessarily become completelyconstant. However, even in such a case, by setting the maximum voltagevalue Vmax to a little bit lower value while setting the minimum voltagevalue Vmin to a little bit higher value, it becomes possible to detectthe fact that the voltage value reaches the peak with a sufficientaccuracy even if the amplitude of the beam 104 includes a minutevariation.

In the case (the case of NO in the step S202) in which the peak of thevoltage value generated in the controlling piezoelectric device 110 isnot detected, the steps S200 through S202 are repeated until the peak ofthe voltage value generated in the controlling piezoelectric device 110is detected. In the case (the case of YES in the step S202) in which thepeak of the voltage value generated in the controlling piezoelectricdevice 110 is detected, the control section 130 a set (step S204) theswitch SW of the resonant circuit (the resonant circuit composed of thecapacitive component Cg of the piezoelectric device 108 and the inductorL) to the ON state. In the present embodiment, the control circuit 136 aof the control section 130 a outputs a control signal to the switch SWto thereby set the switch SW to the ON state.

After the step S204, the control section 130 a starts (step S206) atimer not shown and incorporated in the control circuit 136 a. After thestep S206, the control section 130 a determines (step S208) whether ornot the period half as long as the resonance period T of the resonantcircuit composed of the capacitance component Cg of the piezoelectricdevice 108 and the inductor L. In the present embodiment, the controlcircuit 136 a of the control section 130 a performs the determination inthe step S208. If the control circuit 136 a determines (the case of NOin the step S208) that the period half as long as the resonance period Thas not elapsed, the step S208 is repeated.

If the control section 136 a determines (the case of YES in the stepS208) that the period half as long as the resonance period T haselapsed, the control section 130 a sets (step S210) the switch SW of theresonant circuit to the OFF state. In the present embodiment, thecontrol circuit 136 a of the control section 130 a outputs a controlsignal to the switch SW to thereby set the switch SW to the OFF state.

After the step S210, the control section 130 a repeats the steps S200through S210.

By setting the switch SW of the resonant circuit to the ON/OFF states insuch a manner as described hereinabove, the switch SW can be switchedbetween the ON/OFF states at appropriate timings in accordance with themovement of the beam 104, and therefore, it becomes possible toefficiently generate the electrical power using the power generationunit 100.

It is preferable that the power generation amount of thepower-generating piezoelectric device 108 is larger than the powergeneration amount of the controlling piezoelectric device 110. It issufficient for the controlling piezoelectric device 110 to assure thepower generation amount necessary for the control, and by setting thepower generation amount of the power-generating piezoelectric device 108to be large, the power generation unit 100 can efficiently generate theelectrical power. Further, by setting the power generation amount of thecontrolling piezoelectric device 110 to the minimum power generationamount necessary for the control, the displacement resistance of thebeam 104 due to the controlling piezoelectric device 110 is reduced, andthe power generation efficiency is improved.

D. FOURTH EMBODIMENT

In the explanation of the power generation unit 100 according to thethird embodiment described above, it is assumed that a singlecontrolling piezoelectric device 110 is disposed. However, it is notnecessarily required to provide the single controlling piezoelectricdevice 110, but a plurality of such controlling piezoelectric devicescan also be provided. Hereinafter, a fourth embodiment with such aconfiguration will be explained. The constituents substantially the sameas those of the second and third embodiments will also be attached withthe same reference numerals in the fourth embodiment, and the detailedexplanation therefore will be omitted.

FIG. 17 is a circuit diagram showing an electrical structure of thepower generation unit 100 according to the fourth embodiment providedwith a plurality of controlling piezoelectric devices. The mechanicalstructure of the power generation unit 100 according to the fourthembodiment is the same as the structure shown in FIGS. 13A and 13B. Thepower generation unit 100 according to the fourth embodiment is providedwith two controlling piezoelectric devices 110, 114.

Similarly to the second embodiment, in the power generation unit 100according to the fourth embodiment, if the power-generatingpiezoelectric device 108 has the maximum installable length and widthwith respect to the beam 104, the amount of power generation of thepower-generating piezoelectric device 108 becomes large, and if thecontrolling piezoelectric devices 110, 114 have the minimum installablewidth (length in the direction along the shorter dimension of the beam104), the displacement resistance of the beam 104 due to the controllingpiezoelectric devices 110, 114 is reduced, and therefore, high powergeneration efficiency is obtained.

In the power generation unit 100 according to the fourth embodiment, bydisposing the two controlling piezoelectric devices 110, 114 at thepositions shifted toward the both sides of the beam 104 in the widthdirection thereof, the controlling piezoelectric devices 110, 114 canset the switch SW to the ON/OFF states at appropriate timings even inthe case in which the beam 104 generates the displacement differentbetween the vertical and horizontal positions, and therefore, the powergeneration unit 100 can be used in a variety of situations.

The first controlling piezoelectric device 110 is expressed as acombination of a current source and the capacitor Cs1 for storingcharges, and the second controlling piezoelectric device 114 isexpressed as a combination of a current source and the capacitor Cs2 forstoring charges. The first electrode 110 a and the second electrode 110b of the first controlling piezoelectric device 110 are connected to thecontrol section 130 a, and the first electrode 114 a and the secondelectrode 114 b of the second controlling piezoelectric device 114 arealso connected to the control section 130 a.

In the control section 130 a, either one of a pair of first electrode110 a and the second electrode 110 b and a pair of first electrode 114 aand the second electrode 114 b is selected, and then the voltage valueof the selected one of the piezoelectric devices 110, 114 is detected tothereby control the switch SW. For example, the amount of powergeneration has previously been measured when installing the powergeneration unit 100 in the case of generating the electrical power whiledetecting the voltage value of the piezoelectric device 110 and in thecase of generating the electrical power while detecting the voltagevalue of the piezoelectric device 114. Either one of the cases with alarger amount of power generation has previously been selected using aswitch or the like provided to the control circuit 136 a. If either oneof the piezoelectric device 110 and the piezoelectric device 114 haspreviously been selected in such a manner as described above, byperforming the switch control process described above with reference toFIG. 16, the ON/OFF control of the switch SW can be performed.

Although the two piezoelectric devices 110, 114 generate broadly similarvoltage waveforms, a slight difference in voltage waveform and magnitudeof the voltage amplitude can occur due to the factors such as thestructure of the beam 104 and the production tolerance. Further, if thedifference occurs in the voltage waveform, there is a possibility ofcausing the difference in power generation amount, and if the differenceoccurs in the magnitude of the voltage amplitude, there is a possibilityof controlling the switch SW at a more appropriate timing when using oneof the piezoelectric devices (having higher sensitivity as a sensor)with which a larger voltage amplitude can be obtained. Therefore, byhaving previously measured the power generation amount in the case ofgenerating the electrical power while detecting the voltage of thepiezoelectric device 110 and in the case of generating the electricalpower while detecting the voltage of the piezoelectric device 114, andthen selecting one with larger power generation amount, it becomespossible to more efficiently generate the electrical power.

In the explanation described above, it is assumed that either one of thepair (the first electrode 110 a and the second electrode 110 b) ofterminals of the piezoelectric device 110 side and the pair (the firstelectrode 114 a and the second electrode 114 b) of terminals of thepiezoelectric device 114 side is selected and used for controlling theswitch SW. However, it is also possible to arrange that the firstelectrode 110 a and the first electrode 114 a are connected to eachother, and the second electrode 110 b and the second electrode 114 b areconnected to each other to thereby detect the electrical potentialdifference (the voltage value) between the first electrode side and thesecond electrode side, and thus, the switch SW is controlled. Whencontrolling the switch SW, the switch control process described abovewith reference to FIG. 16 can be applied.

In consequence of the structure of the beam 104, the environment of theinstallation of the power generation unit 100, and so on, there is apossibility of generating a torsional deformation of the beam 104. Whenthe torsional deformation occurs in the beam 104, there is a possibilitythat the phases of the voltage waveforms generated by the piezoelectricdevices 110, 114 are shifted. However, by disposing the piezoelectricdevices 110, 114 at the positions shifted toward the both sides of thebeam 104 as shown in FIG. 13B, the influences of the torsionaldeformation with respect to the deflection of the beam 104 are reverseto each other. Therefore, by connecting the first electrode 110 a andthe first electrode 114 a to each other, and the second electrode 110 band the second electrode 114 b to each other, the influences of thetorsional deformation of the beam 104 on the piezoelectric devices 110,114 can be canceled out each other. As a result, even in the case inwhich the torsional deformation is caused in the beam 104, the switch SWcan be controlled at appropriate timings without being affected by thetorsional deformation, and therefore, it becomes possible to efficientlygenerate the electrical power.

E. MODIFIED EXAMPLES

A variety of modified examples exist in the first, second, third, andfourth embodiments described above. Hereinafter, these modified exampleswill briefly be explained.

E-1. First Modified Example

In the second and fourth embodiments described above, the explanation ispresented assuming that the two controlling piezoelectric devices 110,114 have the same length as that of the power-generating piezoelectricdevice 108, and the controlling piezoelectric devices 110, 114 aredisposed at the positions shifted toward the both ends of the beam 104in the width direction thereof. However, it is also possible to arrangethat the two piezoelectric devices 110, 114 shorter than a half of thelength of the power-generating piezoelectric device 108 are disposed ata central position of the beam 104 so as to be arranged in a line in thelongitudinal direction.

FIGS. 18A and 18B are explanatory diagrams showing the condition inwhich the power-generating piezoelectric device 108 and the twocontrolling piezoelectric devices 110, 114 are provided to the beam 104of the power generation unit 100 according to the first modifiedexample. FIG. 18A is a plan view thereof viewed from one surface of thebeam 104. FIG. 18B is a plan view thereof viewed from the other surfaceof the beam 104. FIG. 18A shows the condition in which thepower-generating piezoelectric device 108 is disposed, and FIG. 18Bshows the condition in which the two controlling piezoelectric devices110, 114 are disposed.

Depending on the structure of the beam 104 and the environment of theinstallation of the power generation unit 100, there is a possibility ofcausing the case in which the beam 104 is deformed in an undulatingmanner. Then, the parts with large deformation caused by the deflectionof the beam 104 and the parts with small deformation occur along thelongitudinal direction of the beam 104. Therefore, by having disposedthe two short piezoelectric devices 110, 114 at the center of the beam104 in a line along the longitudinal direction as shown in FIG. 18B, andselecting one of the piezoelectric devices generating the currentwaveform or the voltage waveform with a sufficient amplitude, it becomespossible to control the switch SW at appropriate timings even in thecase in which the undulating deformation occurs in the beam 104.

Although the explanation is presented with reference to FIGS. 18A and18B assuming that the two piezoelectric devices 110, 114 are disposed asthe controlling piezoelectric devices, it is also possible to arrangethat three or more piezoelectric devices are disposed.

E-2. Second Modified Example

In the various embodiments or the first modified example describedabove, the explanation is presented assuming that the controllingpiezoelectric device 110 (and the controlling piezoelectric device 114)is disposed on the surface different from the surface provided with thepower-generating piezoelectric device 108. However, it is also possibleto dispose the controlling piezoelectric device 110 on the same surfaceas the surface provided with the power-generating piezoelectric device108.

FIG. 19 is an explanatory diagram showing the condition in which thepower-generating piezoelectric device 108 and the controllingpiezoelectric devices 110 are provided to the same surface of the beam104 of the power generation unit 100 according to a second modifiedexample. In the example shown in FIG. 19, the power-generatingpiezoelectric device 108 and the controlling piezoelectric device 110are disposed on the same surface of the beam 104. The controllingpiezoelectric device 110 has the same length as the piezoelectric device108, but has a width smaller than that of the piezoelectric device 108.As described above, if the controlling piezoelectric device 110 havingroughly the same length is disposed in parallel to the power-generatingpiezoelectric device 108, roughly the same deformations occurrespectively in the piezoelectric device 108 and the piezoelectricdevice 110. Therefore, it becomes possible to accurately detect thetiming at which the deformation direction of the piezoelectric device108 is switched to thereby control the switch SW at an appropriatetiming.

Obviously, in the case of disposing the power-generating piezoelectricdevice 108 and the controlling piezoelectric device 110 on the samesurface of the beam 104, the size (the area) of the power-generatingpiezoelectric device 108 is reduced in accordance with the size of thecontrolling piezoelectric device 110. As a result, as in the variety ofembodiments or the first modified example described above, the powergeneration capacity is degraded compared to the case of disposing thepower-generating piezoelectric device 108 and the controllingpiezoelectric device 110 on the respective surfaces different from eachother. However, as shown in FIG. 19, since the controlling piezoelectricdevice 110 has a small width, it is possible to reduce the decrease inarea of the power-generating piezoelectric device 108 to a relativelysmall value, and the degradation of the power generation capacity canalso be reduced to a relatively low level.

On the other hand, if the power-generating piezoelectric device 108 andthe controlling piezoelectric device 110 are disposed on the samesurface as in the second modified example shown in FIG. 19, thepiezoelectric device 108 and the piezoelectric device 110 can bedisposed in the same process. In contrast, if the power-generatingpiezoelectric device 108 and the controlling piezoelectric device 110are disposed on the respective surfaces different from each other as inthe variety of embodiments and the first modified example describedabove, the process of disposing the piezoelectric device 108 and theprocess of disposing the piezoelectric device 110 must be made separatefrom each other. Therefore, by disposing the power-generatingpiezoelectric device 108 and the controlling piezoelectric device 110 onthe same surface as in the second modified example, it becomes possibleto simplify the manufacturing process of the power generation unit 100.Conversely, in the case of disposing the power-generating piezoelectricdevice 108 and the controlling piezoelectric device 110 on therespective surface different from each other as in the variety ofembodiments and the first modified example described above, since thearea of the power-generating piezoelectric device 108 can be increasedalthough the manufacturing process of the power generation unit 100becomes complicated, it becomes possible to improve the power generationcapacity.

Hereinabove, the explanation is presented assuming that the controllingpiezoelectric device 110 has roughly the same length as thepower-generating piezoelectric device 108, but has a width smaller thanthat of the piezoelectric device 108. However, it is also possible toassume that the controlling piezoelectric device 110 having roughly thesame width as the power-generating piezoelectric device 108 and ashorter length is used, and the piezoelectric devices 108, 110 aredisposed on the same surface of the beam 104.

FIG. 20 is an explanatory diagram showing another configuration of thesecond modified example having the power-generating piezoelectric device108 and the controlling piezoelectric device 110 disposed on the samesurface of the beam 104. In the example shown in FIG. 20, the deformingmember (the beam 104) is configured including an undeformable stationaryend (a connection section to the base 102), and the piezoelectric device110 is disposed at a place closer to the stationary end than thepiezoelectric device 108. In a so-called cantilever such as the beam104, the bending moment of a part increases as the part moves from thetip and comes closer to the base 102, and the deformation amount of thebeam 104 per unit length also increases in conjunction therewith.Therefore, by disposing the controlling piezoelectric device 110 in thevicinity of the base 102, the sensitivity as the sensor is improved, andthe width of the controlling piezoelectric device 110 can be reducedaccordingly. As a result, since the area of the power-generatingpiezoelectric device 108 can be increased, it becomes possible tosuppress the degradation of the power generation capacity caused bydisposing the power-generating piezoelectric device 108 and thecontrolling piezoelectric device 110 on the same surface.

E-3. Third Modified Example

In the second modified example described above, the explanation ispresented assuming that the power-generating piezoelectric device 108and the controlling piezoelectric device 110 are disposed on the samesurface of the beam 104, and no other controlling piezoelectric devicethan the controlling piezoelectric device 110 is disposed. However, evenin the case in which the power-generating piezoelectric device 108 andthe controlling piezoelectric device 110 are disposed on the samesurface of the beam 104, it is possible to arrange that a plurality ofcontrolling piezoelectric devices is disposed.

FIG. 21 is an explanatory diagram showing a third modified examplehaving the power-generating piezoelectric device 108 and a plurality ofcontrolling piezoelectric devices 110, 114 disposed on the same surface.In the example shown in the drawing, the controlling piezoelectricdevices 110, 114 narrower in width than the power-generatingpiezoelectric device 108 and less than half as short in length as thepower-generating piezoelectric device 108 are arranged in a line anddisposed in parallel to the piezoelectric device 108.

As described above, depending on the structure of the beam 104 and theenvironment of the installation of the power generation unit 100, thereis a possibility of causing the case in which the beam 104 is deformedin an undulating manner. Then, the parts with large deformation causedby the deflection of the beam 104 and the parts with small deformationoccur along the longitudinal direction of the beam 104. Therefore,depending on the position where the controlling piezoelectric devices110, 114 are disposed, the case in which sufficient sensitivity (thecurrent value to be detected) fails to be obtained can occur. Therefore,by having disposed the two short piezoelectric devices 110, 114 in aline along the longitudinal direction of the beam 104 as shown in FIG.21, and selecting one of the piezoelectric devices which can providesufficient sensitivity, it becomes possible to control the switch SW atappropriate timings even in the case in which the undulating deformationoccurs in the beam 104. Obviously, the controlling piezoelectric devicesare not limited to the two piezoelectric devices 110, 114, but it isalso possible to arrange that three or more piezoelectric devices aredisposed.

In the third modified example shown in FIG. 21, the explanation ispresented assuming that the two short controlling piezoelectric devices110, 114 are disposed in a line in the longitudinal direction of thebeam 104. Therefore, it results that the two short controllingpiezoelectric devices 110, 114 are disposed on one side with respect tothe power-generating piezoelectric device 108. In contrast, it is alsopossible to arrange that the controlling piezoelectric devices 110, 114having roughly the same length as that of the power generatingpiezoelectric device 108 and a narrower width are disposed on both sidesof the power-generating piezoelectric device 108.

FIG. 22 is an explanatory diagram showing another configuration of thethird modified example having the power-generating piezoelectric device108 and a plurality of controlling piezoelectric devices 110, 114disposed on the same surface. As described above, in consequence of thestructure of the beam 104, the environment of the installation of thepower generation unit 100, and so on, there is a possibility ofgenerating a torsional deformation of the beam 104. When the torsionaldeformation occurs in the beam 104, there is a possibility that thephases of the current waveforms generated by the piezoelectric devices110, 114 are shifted, and it becomes unachievable to switch the switchSW at an appropriate timing. However, the piezoelectric devices 110, 114having a narrow width are disposed on both sides of the power-generatingpiezoelectric device 108 as shown in FIG. 22, the power generationamount when controlling the switch SW using the current values of therespective piezoelectric devices 110, 114 is measured, and one of thepiezoelectric devices 110, 114 with larger power generation amount isselected. According to this procedure, the degradation of the powergeneration capacity can be reduced even in the case in which thetorsional deformation is generated in the beam 104.

Although the embodiments and the modified examples are explainedhereinabove, the invention is not limited to the embodiments and themodified examples described above, but can be put into practice invarious forms within the scope or the spirit of the invention.

For example, in the embodiments and the modified examples describedabove, the explanation is presented assuming that the piezoelectricdevices 108, 110 are attached to the beam 104 having the cantileverstructure. However, the piezoelectric devices 108, 110 can be attachedto any member providing the member is easily deformed in a repeatedmanner due to a vibration or the like. For example, the piezoelectricdevices 108, 110 can be attached to a surface of a thin film, or to aside surface of a coil spring.

In the embodiments and the modified examples described above, thepiezoelectric constant of the power-generating piezoelectric device 108can also be higher than the piezoelectric constant of the controllingpiezoelectric devices 110, 114. It is sufficient for the controllingpiezoelectric devices 110, 114 to assure the power generation amountnecessary for the control, and by setting the piezoelectric constant ofthe power-generating piezoelectric device 108 to be large, the powergeneration amount of the power generation unit 108 can be increased. Asa result, it becomes also possible to reduce the area of thepower-generating piezoelectric device 108, miniaturization of the beam104 can be achieved.

In the embodiments and the modified examples described above, althoughthe power generation amount of the power-generating piezoelectric device108 is set to be larger than the power generation amount of thecontrolling piezoelectric devices 110, 114, in comparison between thepower-generating piezoelectric device 108 and the controllingpiezoelectric devices 110, 114, the power generation amount can bedifferent due to the piezoelectric constant, the total area of the partcapable of generating the electrical power, the thickness, and so on. Itis also possible to make the power generation amount of thepower-generating piezoelectric device 108 larger than the powergeneration amount of the controlling piezoelectric devices 110, 114 byinstalling the piezoelectric device in accordance with the displacementcharacteristics of the beam 104 in the part of the beam 104 with higherdisplacement frequency or larger displacement amount to thereby make thepower generation amount different.

In the embodiments and the modified examples described above, two ormore power-generating piezoelectric devices can also be provided. Thelarger the plural number of the power-generating piezoelectric devicesis, the more the power generation amount becomes. Therefore, byproviding a plurality of power-generating piezoelectric devices, ahigher voltage than the voltage of the controlling piezoelectric deviceis generated from the power-generating piezoelectric device used forsupplying the charge to the outside, and the power generation capacityof the power generation unit can be improved.

Since the power generation unit according to the invention generatespower in accordance with the vibration or the transportation, byinstalling the power generation unit on a bridge, a building, or apossible landslide place, it is also possible to generate electricalpower at the time of disaster such as an earthquake, and to supply theelectricity to a network device such as an electronic apparatus at onlythe time of need (disaster).

The power generation unit according to the embodiment of the inventioncan be miniaturized, and can therefore be installed in every apparatusbesides the electronic apparatus. For example, by applying the powergeneration unit according to the embodiment of the invention to atransportation device such as a vehicle or an electric train, it is alsopossible to generate power by the vibration due to the transportation,and to supply the electrical power efficiently to the equipment providedto the transportation device.

In this case, in order to cope with all of the vibrations, it is alsopossible to incorporate a plurality of power generation units 100different in length of the beam 104 and weight of the mass 106 in thetransportation device. For example, it is also possible to constitute apower generating unit having the plurality of power generation units 100fixed to the base 102 common to the power generation units 100.

It is also possible to incorporate the power generation unit accordingto the embodiment of the invention in an electronic apparatus such as aremote controller instead of the battery.

Further, the power generation unit according to the embodiment of theinvention can be provided with the same shape as, for example, a buttonbattery or a dry-cell battery, and can also be used in generalelectronic apparatuses instead of being installed in a specificelectronic apparatus or the like. In this case, since it is possible tocharge the capacitor by a vibration, the power generation unit can beused as a battery even in the time of disaster with electricity lost.Since the life thereof is longer than that of a primary cell, reductionof environmental load can be achieved in terms of a life cycle.

The invention includes configurations (e.g., configurations having thesame function, the same way, and the same result, or configurationshaving the same object and the same advantages) substantially the sameas those described in the embodiment section. The invention includesconfigurations obtained by replacing non-essential parts of theconfigurations described in the embodiment section. The inventionincludes configurations providing the same functions and the sameadvantages or configurations capable of achieving the same object asthose of the configurations described in the embodiment section. Theinvention includes configurations obtained by adding technologies knownto the public to the configurations described in the embodiment section.

This application claims priority to Japanese Patent Application No.2011-219333 and Application No. 2011-218989 filed on Oct. 3, 2011, theentirety of which is hereby incorporated by reference.

What is claimed is:
 1. A power generation unit comprising: a firstpiezoelectric device; a second piezoelectric device; an inductorelectrically connected to the first piezoelectric device; and a controlsection configured to electrically connect the first piezoelectricdevice and the inductor when a current generated in the secondpiezoelectric device is equal to or higher than a predetermined currentlevel.
 2. The power generation unit according to claim 1, wherein thecontrol section electrically connects the first piezoelectric device andthe inductor at a timing when a deformation direction of the firstpiezoelectric device and the second piezoelectric device is switched,and then electrically disconnects at a timing when a predeterminedperiod has elapsed.
 3. The power generation unit according to claim 1,wherein the control section includes a capacitor connected in parallelto the second piezoelectric device, and a current detect circuit adaptedto detect a current flowing in the capacitor.
 4. The power generationunit according to claim 1, further comprising: a deforming unit at whichthe first piezoelectric device and the second piezoelectric device aredisposed, wherein the first piezoelectric device is provided to a firstsurface of the deforming member, and the second piezoelectric device isprovided to a second surface of the deforming member different from thefirst surface.
 5. The power generation unit according to claim 1,further comprising: a deforming unit on which the first piezoelectricdevice and the second piezoelectric device are disposed, wherein thefirst piezoelectric device and the second piezoelectric device areprovided to a same surface of the deforming member.
 6. The powergeneration unit according to claim 1, further comprising: a deformingunit on which the first piezoelectric device and the secondpiezoelectric device are disposed, wherein the deforming member has anundeformable stationary end, and the second piezoelectric device isdisposed at a place closer to the stationary end of the deforming memberthan a place in the deforming member at which the first piezoelectricdevice is disposed.
 7. An electronic apparatus comprising the powergeneration unit according to claim
 1. 8. A transportation devicecomprising the power generation unit according to claim
 1. 9. A methodof controlling a power generation unit including a first piezoelectricdevice, a second piezoelectric device, an inductor electricallyconnected to the first piezoelectric device; and a switch disposedbetween the first piezoelectric device and the inductor, the methodcomprising: detecting a current generated in the second piezoelectricdevice; and connecting the first piezoelectric device and the inductorelectrically to each other via the switch based on the detection resultof the current.
 10. A power generation unit comprising: a firstpiezoelectric device; a second piezoelectric device adapted to generateelectrical power, an amount of which is smaller than an amount ofelectrical power generated by the first piezoelectric device; aninductor electrically connected to the first piezoelectric device; and acontrol section configured to electrically connect the firstpiezoelectric device and the inductor when a voltage generated in thesecond piezoelectric device is equal to or higher than a predeterminedvoltage level.
 11. The power generation unit according to claim 10,wherein a piezoelectric constant of the first piezoelectric device ishigher than that of the second piezoelectric device.
 12. The powergeneration unit according to claim 10, wherein a generating area of thefirst piezoelectric device is larger than that of the secondpiezoelectric device.
 13. The power generation unit according to claim10, wherein the number of the first piezoelectric devices is plural. 14.The power generation unit according to claim 10, further comprising: adeforming unit on which the first piezoelectric device and the secondpiezoelectric device are disposed, wherein the first piezoelectricdevice is provided to a first surface of the deforming member, and thesecond piezoelectric device is provided to a second surface of thedeforming member different from the first surface.
 15. The powergeneration unit according to claim 10, further comprising: a deformingunit on which the first piezoelectric device and the secondpiezoelectric device are disposed, wherein the first piezoelectricdevice and the second piezoelectric device are provided to a samesurface of the deforming member.
 16. The power generation unit accordingto claim 10, wherein the first piezoelectric device and the secondpiezoelectric device are equal in length in a longitudinal direction toeach other.
 17. The power generation unit according to claim 10, furthercomprising: a deforming unit on which the first piezoelectric device andthe second piezoelectric device are disposed, wherein the deformingmember has an undeformable stationary end, and the second piezoelectricdevice is disposed at a place closer to the stationary end of thedeforming member than a place of the deforming member at which the firstpiezoelectric device is disposed.
 18. An electronic apparatus comprisingthe power generation unit according to claim
 10. 19. A transportationdevice comprising the power generation unit according to claim 10.